The Amalga-Beast was created by Namor the Sub-Mariner, a lesser known Marvel character who explains the process in the above frame from Avengers 1#40 (1967). Namor himself is an aquatic mutant, and the Amalga-Beast was "formed by combining the bodies of various of [his] finny subjects."
You can't see too well from these frames, but it looks as though these "finny subjects" included at least a turtle, a crab, an octopus and a shark. This clever person made a model which shows more details:
Coincidentally (segue alert), Namor appears in another series of comics starring Grotesk, the super-villain and all-round subterranean manthing who just happens to be this blog's namesake. Who knew Marvel were so interested in the grotesque? If I get around to scanning the images I might post about Grotesk sometime soon. He never gets much publicity, for some reason.