Around here, nearly every unwanted puppy at the pound is half pit bull. Cats screw all the time too. Howcum you never see a fish that is say half shark and half guppy?
How about an electric eel bred with a cod (for the oil)? That would be a hybrid, right?
Basically, different fishes are different species. Dogs are all Canis lupus (as are wolves). So a wolf and a poodle can procreate, but a shark and a guppy can’t.
Sharks and guppies are too distantly related, of course. All domestic dogs are the exact same species, so they can breed, given logistical difficulties between some breeds.
ETA: Some closely related species can breed. Usually the offspring is sterile, but not always.
Lots. Here is the hybrid striped bass.
How about shellfish?