Landlord suddenly say cannot keep cat, but I already have one. What to do?

First thing, actually read your tenancy agreement, not what the landlord says verbally. If there's a no-pets clause and you signed it, technically you're in breach, so approach it as negotiation not a fight. What worked for my friend: she offered a small extra deposit for potential pet damage, showed the unit was spotless, and got the landlord to add a written addendum allowing one neutered indoor cat. Landlords worry about smell, scratched doors and fleas, so address those directly, show you neuter, litter train and clean. If the landlord still refuses and enforces the clause, you may have to comply or move at end of tenancy, but they generally can't just throw you out mid-term without following the agreement. If it gets messy, the Tribunal for Housing or legal aid can advise, but 90 percent of the time a calm chat and a deposit top-up solves it.