My pets are full? Can i sell pets? How to remove them?

Bottom right, click pets, scroll to the pet you dont want, click the free button.

Depending on the number of stars you get some rewards back.

1-2 stars: NUTHIN

3 Stars: 1/3 x Large EXP Pet Pots

4 Stars: Kindling/Spiritual Fruit/Spirit Crystal/???

5 Stars: 1 x Mystery Summon (Regardless of VIP)

Wait.. why are you releasing 5* pets? Sometimes you get extra pets. I have 3 archangels -.-