Inhumans (2000) #2's story had Ronan explaining that the phenotypes of each or many of the Inhumans was based on or matched that of other extraterrestrials. If this is true, the Pelagians would seem likely to be the template for Triton.
    Thanks to David Zuckerman for both reminding me of the issue and providing the image to the right------>

First, the Pelagian race and the planet Pelagia have names that are very appropriate and meaningful...to people from Earth. This is because, by an INCREDIBLE coincidence, the word "pelagic" means "of, relating to, or living or occurring in the open sea." It is a less-used synonym of "oceanic" and comes from the Ancient Greek pelagos which means "open sea."

Second, as you noted, the reason why the Inhumans transformed into various non-human forms after exposure to the Terrigen Mist was explained to them by Ronan in issue #2 of the 2000 Inhumans mini-series. I thought it was a clever twist to reveal that those forms were actually reproductions of those of various alien races and that the Kree had always planned for the Inhumans to be able to duplicate the powers possessed by those races and to be able to infiltrate those races if their Kree masters needed. Sadly, this explanation has probably been overwritten by Jonathan Hickman's creation of the Universal Inhumans, which seemingly made Earth's Inhumans one of over a hundred different races upon whom the Kree experimented. Hickman also introduced the idea that the experiment that created the Universal Inhumans was performed about 300,000 years ago, contradicting the far more recent (i.e., tens of thousands of years ago) origin of Earth's Inhumans, a contradiction that hasn't yet been resolved (or even been addressed).

Third, evidence for the (apparent) validity of Ronan's claim was presented in that same issue, when the Shi'ar Captain P'kkrd mentioned that it was baffling that some of the marauders who had just attacked his ship resembled races that had been extinct for millennia, and Ronan told Gorgon that the species that was his mold no longer existed and/or was no longer a threat.

Fourth, it's worth noting that the form possessed by Triton at the time of the Secret Invasion: Inhumans mini-series was notably different from his original form. This difference has never received an in-universe explanation. I think that Triton made have had yet another appearance in the past but, if so, I can't remember where I saw it. 

Finally, Triton was one of the Inhumans chosen to infiltrate the Shi'ar warrior candidates, and the Shi'ar who "rescued" him from within their damaged ship referred to him as "the Mendarrian." This suggests that Triton's original form, which he still possessed during that 2000 Inhumans mini-series, was identical to that of a race known to the Shi'ar as the Mendarrians, a race of water-breathers who could survive in air for a time.

This raises an interesting question: Are "Mendarrians" and "Pelagians" different names for a single species who may (or may not) have originated on the planet Pelagia? Or did Triton, in his two forms, resemble two similar but not identical alien races of water-breathers?
Don Campbell

(Secret Invasion: Inhumans#3 (fb) - BTS) - As part of their plan to maintain the security of their decryption systems, the Skrulls buried a beacon in the deepest trenches of Pelagia.

    For unrevealed reasons -- whether Skrull order or just because the Pelagian ruler ordered it so for reasons unspecified -- the Pelagians were not allowed to approach the Skrull beacon.

(Secret Invasion: Inhumans#3 - BTS) - To locate Black Bolt, who was a captive of the Skrulls, the Inhuman Karnak determined that three items could be used to breech Skrull decryption security: (1) On Earth, a Skrull commander held prisoner by Thundra and her tribe of Femizons; (2) On "Rigel 3" (presumably New Rigel-3), a Rigellian Recorder that had cracked the Skrull communications algorithm; (3) On Pellagia, the Skrull beacon.

(Secret Invasion: Inhumans#3 - BTS) - Lockjaw transported the aquatic Inhuman Triton, who was virtually identical to a Pelagian, to Pelagia to recover the Skrull beacon.

(Secret Invasion: Inhumans#3) - As Lockjaw swam away, Triton dove beneath the surface and headed for the trench containing the beacon, but he was swiftly confronted by a legion of Pelagians.

(Secret Invasion: Inhumans#4) - After allowing the legion to lead him into the vicinity of the beacon, Triton incapacitated the entire Legion -- after they confronted him with intent of stopping him -- and approached the beacon, the communications hub of which he uplinked to Karnak's computers.

    Lockjaw then arrived and teleported the Triton and himself back to Karnak.

(Secret Invasion: Inhumans#4) - Karnak synched the uplinked Skrull communications hub on Pellagia with the Rigellian Recorder obtained by Karnak and Gorgon, and then Crystal forced the Skrull commander she and Medusa had recovered to reveal the password.

(Secret Invasion: Inhumans#3 (fb) - BTS) - As part of their plan to maintain the security of their decryption systems, the Skrulls buried a beacon in the deepest trenches of Pelagia.

    For unrevealed reasons -- whether Skrull order or just because the Pelagian ruler ordered it so for reasons unspecified -- the Pelagians were not allowed to approach the Skrull beacon.

(Secret Invasion: Inhumans#3 - BTS) - To locate Black Bolt, who was a captive of the Skrulls, the Inhuman Karnak determined that three items could be used to breech Skrull decryption security: (1) On Earth, a Skrull commander held prisoner by Thundra and her tribe of Femizons; (2) On "Rigel 3" (presumably New Rigel-3), a Rigellian Recorder that had cracked the Skrull communications algorithm; (3) On Pellagia, the Skrull beacon.

(Secret Invasion: Inhumans#3 - BTS) - Lockjaw transported the aquatic Inhuman Triton, who was virtually identical to a Pelagian, to Pelagia to recover the Skrull beacon.

(Secret Invasion: Inhumans#3) - As Lockjaw swam away, Triton dove beneath the surface and headed for the trench containing the beacon, but he was swiftly confronted by a legion of Pelagians, the female leader of which asked him, "You there...what are you doing?"

(Secret Invasion: Inhumans#3 (fb) - BTS) - Triton allowed the Pelagians to take him captive, wrapping his wrists in some sort of seaweed rope.

(Secret Invasion: Inhumans#3) - The Pelagians led Triton by a tether of the same seaweed that bound his wrists.

(Secret Invasion: Inhumans#4) - The leader of the legion told Triton that he looked like them but was not a Pelagian, and she asked him if he was a spy. When he replied that he was there to find the communication device that was a hub for the Skrull Empire, she responded that that sounded like something a spy would say. When he asked if the Skrull beacon was ahead of them, the leader grabbed him by the neck and instructed Triton that he was in no position to demand anything, he replied that he needed the device to save his king. She informed him that his king was out of luck as no Pelagian was allowed near it. Breaking free of his bonds, Triton reminded her that he was not Pelagian.

(Secret Invasion: Inhumans#4 (fb) - BTS) - Triton incapacitated the entire Legion and approached the beacon.

(Secret Invasion: Inhumans#4) - Lying at the ground by Triton's feet as he accessed the beacon, the leader asked how Triton -- who was exactly like them -- had bested their entire legion. As he uplinked the Skrull commmunications hub, Triton replied that he had endured spending most of his time on land to be with his family despite the strain it was to him.

    Lockjaw then arrived and teleported the two of them back to Karnak.

(Secret Invasion: Inhumans#4) - Karnak synched the uplinked Skrull communications hub on Pellagia with the Rigellian Recorder obtained by Karnak and Gorgon, and then Crystal forced the Skrull commander she and Medusa had recovered to reveal the password.

--Secret Invasion: Inhumans#3

(Secret Invasion: Inhumans#3 (fb) - BTS) - As part of their plan to maintain the security of their decryption systems, the Skrulls buried a beacon in the deepest trenches of Pelagia.

    For unrevealed reasons -- whether Skrull order or just because the Pelagian ruler ordered it so for reasons unspecified -- the Pelagians were not allowed to approach the Skrull beacon.

(Secret Invasion: Inhumans#3 - BTS) - To locate Black Bolt, who was a captive of the Skrulls, the Inhuman Karnak determined that three items could be used to breech Skrull decryption security: (1) On Earth, a Skrull commander held prisoner by Thundra and her tribe of Femizons; (2) On "Rigel 3" (presumably New Rigel-3), a Rigellian Recorder that had cracked the Skrull communications algorithm; (3) On Pellagia, the Skrull beacon.

(Secret Invasion: Inhumans#3 - BTS) - Lockjaw transported the aquatic Inhuman Triton, who was virtually identical to a Pelagian, to Pelagia to recover the Skrull beacon.

(Secret Invasion: Inhumans#3) - As Lockjaw swam away, Triton dove beneath the surface and headed for the trench containing the beacon, but he was swiftly confronted by a legion of Pelagians.

(Secret Invasion: Inhumans#4) - After allowing the legion to lead him into the vicinity of the beacon, Triton incapacitated the entire Legion -- after they confronted him with intent of stopping him -- and approached the beacon, the communications hub of which he uplinked to Karnak's computers.

    Lockjaw then arrived and teleported the Triton and himself back to Karnak.

(Secret Invasion: Inhumans#4) - Karnak synched the uplinked Skrull communications hub on Pellagia with the Rigellian Recorder obtained by Karnak and Gorgon, and then Crystal forced the Skrull commander she and Medusa had recovered to reveal the password.

--(Mentioned) Secret Invasion: Inhumans#3; (shown) Secret Invasion: Inhumans#4