Thats an easy one to remember. Houstier literally sounds like house animal to me so thats how I remember it. An animal that lives in the house is a pet. So it makes sense to me lol.
Hope that helps ;)
I thought Tier was pet and then it said it was animal or beast, so the second time it came up I translated Tier to beast and it said it was just animal. Is beast not a good substitution?
It = Es; 'Das' is more definite. Example: - What's that noise?! - That's an animal. Although you might use 'it' in english, german needs a 'das' there.