The seahorses/dragon types are pretty self explanatory, with the whole Sea Dragon being a type of sea horse which is his lusus and also sea horses are substitutes for dragons in some fiction. Gyarados is the knockoff dragon we never got to see thru. But its huge and mean and right up Eridans dark alley (ha get it).
Cloyster is a badass and used in tournaments still so ofc he would pick the ice/water, big purple ball of anger with a horn on its head. And if you count Eridans fucking obsession with material goods, the whole “pearl in a clam” thing is just even more obvious.
Depending on who he’s fighting he’ll bust out the Mega Gyarados or the Mega Absol.
Also, notice how his lineup isn’t inherently all water types.
Because Eridan Ampora, proud seadweller, has absolutely no business in actually going into the ocean. Like, ever. He just stands around and tells ppl how bad they are for being on the surface and yet????? ?????? ? ? .?? ???