Why can't it be "My horse is one animal"? It was listed in a multiple choice question, and I chose it alongside "My horse is an animal."
For example, you could say "My horse is one animal out of many that I own." Sure it sounds weird and unnatural, but so do many Duolingo sentences so I figured it would be an answer.
Mein is used for masculine and neuter when nominative, in which case meine is used for feminine and plural. I was really confused as well, I thought mein was for neuter, meine for feminine and meinen for masculine, but as it turns out that only happens when accusative case is used. x