So I was sure I'd given my pet an r99 toy to play with before, but not 100% certain, so I gave my pet a to play with and the achievement still hasn't granted, even after going on the achievements page to try and trigger it :-(

Is the item still in your inventory? If so, that means the pet didn't actually play with it. If your pet is happy already they won't play with toys.

Oh :P Yes, it's still there. The pet is at full happiness, oops. I didn't know that! Thanks for telling me, I guess I'll try again with a different pet.

If your pet has a minion they will remain permanently happy. They're usually always happy if you have them in the lodge or at the inn. Gotta find an unhappy pet. :) When it's officially played with the item will disappear.

Yep, it was my battle pet so he always has a minion to keep the happiness full.