I've lost 2 cats - one was euthanized and went through communal cremation. The other passed away while we were out of state, one of our friends was checking in on the pets and found him. If I could go back in time, I would have asked for him to be taken to the vet for communal cremation; as it was, I didn't have much say in the matter and the cat ended up being bagged up and put in the river (and I can't believe I just admitted that).
Any fish I've had got flushed. Small animals (hamsters, gerbils, mice, rats) get put in a small box, taped up securely, and put in the trash. We've always rented and burying isn't an option. Soon we'll own a house and bury any pets. My husband is really attached to one of our cats, I'm not sure what will happen when he passes on, but probably be cremated and keep the ashes.