A gateway cat is a cat that leads its owner to get multiple cats. It starts when someone adopts or buys a single cat. Eventually this person will get another cat to keep the original cat (the gateway cat) company or some other lame excuse. Before you know it, the individual has 40 something cats, is single, and appears on the show "Animal Hoarders".
Pam "I'd like to get a pet. Should I get a cat?"
Amy "NO! It isn't just a cat, it's a gateway cat. You can't have just one. Why don't you get a rabbit instead?"