The Bull Beastie is a new crystal critter that you can get from the Omega Beet Mine.
First you go to The Crystal Catacombs to enter the Elder Vein Mines (don't forget your Mining Helmet), there you must collect all three pieces of the Corrupt Crystal Dampener.
After you have found them, make your way through all the mines again to get to the Omega Beet Mine, where you will find the Corrupt Crystal Core.
Click on the most middle crystal critter (this is the Bull Beastie) and use the Corrupt Crystal Dampener to get it.
NOTE: Kan the Geomancer in The Explorer's Camp can reanimate the critter for you.
NOTE: Make sure to feed it a cake emote every once and a while!