Mushroom King
The Mushroom King is an EXCLUSIVE pet that was sold in the Exclusive Shop for a brief period of time. It was introduced in the Haunted Update. It had a price of 1,299 Robux.
The latest Pet Simulator X value list for Mushroom King can be found below:

Regular
💎 54,000,000,000 (54b)

Golden
💎 Not Available

Rainbow
💎 Not Available

Dark Matter
💎 Not Available