Legal Question in Real Estate Law in Massachusetts

Re: Getting Rid of Dog

It is not a valid reason to evict someone for having the pet that the landlord previously agreed to have there. If you are a tenant-at-will without a lease, though, the landlord can evict for no reason at all, really. He can change his mind about having you there and it would be up to you to prove that it was for some illegal reason.

The best illegal reason for an eviction is if you've reported sanitation or safety violations in writing which have not been fixed (and you can prove you did so) while you were paid up on the rent. That reason holds at least until the violations are corrected and even sometimes another 6 months under a "retaliation" statute.

I'm not sure that changing his mind suddenly about pets is illegal, frankly.

I believe, however, that your having to take on another pet, without his prior permission, however, is a transgression for which he would be entirely justified in asking you to leave.