Correct, it's the raid Naxx in Dragonblight. You don't need to be in a raid to enter it anymore, a party works just fine.
My friend was a lvl 90 Frost DK and I was a lvl 86 resto shamman and we were able to clear the entire instance.
The bone spider pet dropped and the . . . the one that calls in the spores. . .just a sec, let me look it up. . . Fungal Abomination, that's it. Also dropped. The Stitched Pup did not.
The hardest fights in the instance was Stitches (still hits hard) and the Instructor (hits very hard) - and the last boss (which you don't need for the pets, but we completed anyway is still going to kill anyone who gets caught in a frost trap.
The pets are cute, but it's annoying to me that they're all undead. Spider should be beast with maybe undead attacks. Fungal Abomination should be elemental, in my opinion.