Of the fae, but please not disney style. As a worshipper of the Goddess of the Fae Danu, it's saddening that the Fae are always depicted so silly like when The Good Neighbors are extremely mischievous even deadly if you mess up deals with them.

Puppymonkeybabies! Giggles!
Loving Hugs Talia

All the suggestions so far are excellent and I especially like the fey ideas. Here are some of my ideas.
1. Griffins or the flying horses like Pegasus. Either or both could be on the side of good with no rule breaking of their species, even when being playful. Mythological creatures just wanna have fun when they can, so I can see them pulling harmless pranks.
2. Pixies that are taller, closer to human height much like the Elves of LOTR, but always playful and prankish, even when aiding others around them who are quite serious at the time.
3. I have always thought Unicorns as serious and wise, but playful when the opportunity presents itself. However you perceive Unicorns, the new creatures should be the exact opposite in temperate and playfulness of the Unicorns, but on the side of good.
4. Imps, like Morpheus's Fabulous Imp, with a heavy dose of mischief! Your mischievous Imps would be mythological though, and not mutants.
5. Read Robert Asprins' "MYTH" series. There are plenty of ideas for creatures and mayhem in them and full of Puns! Just what you need for the Unicorn 'silliness.'