The Huge Forest Wyvern is an EXCLUSIVE pet that was available from the Exclusive Pets Egg with a 2% hatch rate. It was released in the Glitch Update. This is actually the Huge version of the Nature Dragon pet.
The latest Pet Simulator X value list for Huge Forest Wyvern can be found below:
💎 348,000,000,000 (348b)
💎 748,000,000,000 (748b)
💎 1,000,000,000,000 (1t)
💎 Not Available