What to do when a roommate's pet is a problem
Name the specific problem: barking after midnight, missed cleanup, damage, an unleashed dog, a blocked hallway, or an allergy trigger. Avoid calling the animal bad or the owner careless. Specific facts give the owner something they can solve.
Talk privately and ask for a practical change with a date: more walks, a trainer, a gate, professional cleaning, a different litter-box location, or a sitter. Follow up in writing if the issue affects house rules or the lease.
If there is danger, serious damage, or repeated refusal, document what happened and use the lease or landlord process. Do not retaliate against the animal. The aim is a safe home and a workable decision.