The Horse Mask (or just Horses in general) is a very popular running gag/meme in the Creature universe. The mask is known mainly for its occupation on Sp00n's head, which he wore in vlogs before revealing his face.
The Horse Mask became a common theme with the Creatures when Sp00n first made a vlog in his horse mask (due to not wanting to reveal his face.) Since then, whenever a horse was seen or mentioned the Creatures would go "There's Sp00n." After Sp00n revealed his face at The Creature Panel at RTX 2013, Sp00n of course no longer wore the mask in vlogs. The horse mask can still be seen in Creature videos today, as Aleks wore a horse mask (possibly Sp00n's) to go see Wolverine.
- During his playthrough of Saints Row IV with Nova, (much after revealing his face), Sp00n's character wore a horse mask, a nod to his past persona.