Are adopt an animal charities a con?

A "con"? Not really. Most such "adopt an animal" programs are run by zoos and wildlife sanctuaries that use the money to keep the facility open, to pay for food and vet care, and to expand.

In most cases, the specific animal you adopt doesn't get to keep the money, and it doesn't pay for the welfare of that specific animal, but is pooled with all the other donations for the facility's programs. So if 100 people adopt "Larry the Lemur" and nobody adopts "Stinky the Skunk", both animals still get fed, watered and cared for.