[ May as well say the bones of a dolphin look (vaguely)like a fish, so a dolphin is a fish. ]
There is a fish called a dolphin.. (eye poke)..
LOL! OK, you got me. A dolphin fish IS A FISH. A porpoise, more commonly called a dolphin, is a mammal; although its bones look (vaguely) like a fish.
An opossum or possum is a marsupial, although like a lot of small fuzzy critters - it looks vaguely like a rat.