Spoiler warning~
Thank you! All the boys are a mix of at least one modern fish and one 'living fossil'.
Odus is a mix of a Great White and Frilled shark, his design is mainly inspired off a frilled shark. Vitus is a tigerfish/viperfish and cloelacanth mix. Typus is a whaleshark and false killer whale, but I'm still not 100% sure on his mix yet...
Odus and Vitus see you as a blobfish since they were made by Typus, who was a member of project mermaid, but had a separate lab and ran his own experiments(And he isn't even a scientist, just an occult historian).
So the only one hallucinating is the player since they were programmed that way by the scientists. The scientists of project mermaid wanted mermaids to see eachother as appealing humans and procreate in a human way, so their reproductive organs would stay humanoid-ish.
Also worth noting that Typus has modified your body since you were on the verge of death after being thrown out by the shelter, then dropped you down to one of Tigerfish's dumps - leading you to the beginning of the game. I'm not sure what Typus would do to keep his test subjects fertile, but I'll say he's an unorthodox creep and leave it up to your imagination. XD