I know people are giving some very good examples, but a pet that can easily turn into a hoarding hobby is hamsters. You get one, get super attached, and then three years later whoopsie doodle, the living room is filled floor to ceiling with cages for all twelve of your little dudes.

This is just due to how much space the little guys need. In the wild hamsters will viciously defend miles of land, so bigger cages are always better. As a general rule, an ideal cage should have 900 sq inches of space and be at least 2 feet deep to allow several inches of bedding. So, one little dude will take up at least 12.5 cubic feet of your living room, or .07 cubic smoots for our friends across the pond. This adds up fast, and it can be easy to get in over your head because each individual little dude requires so little cage cleaning per month.