Pet Simulator 99 is part of Roblox The Hunt: Mega Edition, and players must join the game to collect a "The Hunt Token". However, you must complete a specific quest that offers this token as a reward.

Image Source: Roblox Corporation via Twinfinite Lastly, if you put some serious time in Pet Simulator X and have huge pets you left behind, you can transfer them into Pet Simulator 99.