Hippomelon
The Hippomelon is an EXCLUSIVE pet that was only obtainable by completing specific quests during the 2022 April Fools Event.
The latest Pet Simulator X value list for Hippomelon can be found below:

Regular
💎 2,400,000,000 (2.4b)

Golden
💎 Not Available

Rainbow
💎 Not Available

Dark Matter
💎 Not Available