Let's start off with sharks and their sandpaper skin.

If we're not going by porn rules, reptiles and fish don't even have tits, so that's right out.

If we are, some newts have poisonous skin. Also cockroaches or lice. Both would make me want to cross the street to avoid em so god forbid I get a paizuri from either

A teenage Mutant Ninja Turtle.

arachnids and insects, because of their gross appearance

Giraffes, that's it really, there's no explanation it's just giraffes.

A uhh... black widow spider i guess

anything with spikes or scales

Narwhal

The male ones.

but which one could turn you into Squirtle?

fish