Running this query on DataSE I found some tags I think that would make more sense to be pluralized based on the other tags we have.
- aquarium (Note: The similar tag terrariums is plural)
- bearded-dragon
- betta
- budgerigar
- cage
- certificate
- cichlid
- cocker-spaniel
- community-aquarium
- crate (or crate-training)
- crossbreed
- danio
- disaster
- gerbil
- habitat (or remove it)
- herbivore
- maine-coon
- pig
- platy (Should probably be platyfish technically)
- pleco (Should also be the full name plecostomus, rather than the nickname)
- pond
- rat
- recall
- reef-tank
- senior-pet
- skin-condition
- sound
- tetra
- thundershirt (Although there's only one question using this tag, so it might not even be needed.)
- veterinarian
- welsh-collie
- whippet