Creepy Crawlers

Upon use, it allows the three holiday pets, Ghastly Rat, Ghost Maggot, and Spectral Spinner to spawn. In pet battles, any pet that normally can be found in the zone can spawn as secondary pets, so this can be a quicker way to find rare versions of them without having to leave your garrison.

Additionally, it also allows the rare spider Arachnis to spawn. Alliance players can find it near the training dummies, and Horde players will find it between the mine and herb garden. Arachnis can drop the Sack of Spectral Spiders once a day.

Source
Creepy Crawlers can be bought from Izzy Hollyfizzle for. The currency can only be earned during Hallow's End.