It's mob disguise, you can donate to get different sets of mob disguise.
Here's a quote from the donation page:
Mob disguises allow you to appear as a mob ingame. There are 24 disguises to choose from, split into 6 packages Below are the packages can purchase. All packages cost $10. After purchasing your package, you can transform into a mob using /d mobName, such as /d creeper. Use /u to restore normal form. To use the baby disguises, use /d baby mobName Note: You cannot see yourself disguised, but to all others you will appear as the mob.
However, do take note that it's supposed to be 7, not 6 sets. The explanation has yet to be updated.