The law does not require all tenants to be treated the same in regard to pets. If your lease prohibits you from having pets, then you may not have them, even if other tenants do for some reason; if your lease does not prohibit pets, you may have one. When reviewing your lease, you also need to review any other building rules, policies, guidelines referenced in or incorporated into the lease. Note that is not uncommon, when a new pet policy is put into place, to grandfather in pets already in the building, but not to allow new pets, even of then-existing tenants; from what your write, what the building is doing apears to be legal.