Legal Question in Landlord & Tenant Law in Massachusetts

You can ask animal control to come next time and get the dog, since the Tenant says its not his dog. If the Tenant claims the dog, you can then tell them to get rid of it, evict them (assuming the lease says no pets allowed) or you could offer to let them keep the dog and amend the lease and increase the rent and security deposit in relation to the new rent.

If the lease does not prohibit pets, then there is not much you can do before the lease is up.