The Pet Rock was classified as a temporary uncommon pet in Adopt Me! that was only available for free during April Fools' Day 2020.
The Pet Rock was a joke pet that was originally only available during April Fools' Day 2020, but its availability was extended to April 3, 2020, after which it disappeared from every player's inventory and became completely unobtainable.
The pet used to be available outside the Nursery, with an NPC called Burt having a few boxes of Pet Rocks and crossed-out boxes of prices.
The Pet Rock came with the ability to be flyable and rideable and was full grown. There was no way for a player to simultaneously have two Pet Rocks in their possession, as only one could be collected from the pile near the Nursery and it couldn't be traded or given away. The Pet Rock would always disappear out of the player's inventory after they left the game, similar to a Healing Apple, Teacher's Apple, and Marshmallow but this is the only pet that would do that.