UR cat has gone down in rarity. UR dog we don't know the rarity of atm.... but it should be pretty high up there. I'd say it depends on how much value you put on your Zebra. Remember, they're going to slowly grow more common each month. I'm not saying they'll ever not be rare per say, but they are released every month.
UR dog for apple--honestly, your choice. I'd say you could probably get more for an apple right now, the way everyone is freaking out about it, but it sounds pretty fair to trade an UR for a fruit to me. Esp because that said fruit rots