Occasionally, players may put rare huge pets up for auction.
You can purchase it from other players using Diamonds. It is often listed in the Trading Plaza by those who transferred it from Pet Simulator X. Hatch Huge Easter Cat | Really Really Huge xD |...
While the original Huge Easter Cat isn't in the current egg rotation, the offers several new "huge" alternatives you can hatch right now: Occasionally, players may put rare huge pets up for auction