Slimey is a Slime mob who is known to be a temporary pet of Grayson. He was found in an old Iron Myrian castle, which is known as Grayson's temporary home. According to Drake, slimes haven't been seen since before they came through the portal. From the evidence so far, slimes - like ocelots and wolves - can become someone's pet by feeding them a magical food known as Blight Berries. However, they only absorb the berry into their slimy skin, and can eject these at any point in time. Slimy disappeared through a hole in a wall at the beginning of the Time and has only appeared through cameos.
The first notable cameo is in "Campfire," where Slimey is seen hopping in the distance behind Grayson under a tree where Armenbrine is spying on them.
The second cameo is in "Catacombs (Part 1)," where a person is seen walking down the hall before shapeshifting into Slimey. Grayson notices him a second before Slimey enters a hole.
The third cameo is in "The Haunted: Episode 12 Trailer," where Slimey is seen jumping across an area with ruined buildings.