Ghastly Rat
Source: World Event. By far the best way to spread the plague of undeath. Undead type.
The Ghastly Rat is a Hallow's End battle pet introduced in 2015 that spawns inside your Draenor Garrison after applying the 
Creepy Crawlers decoration.
Guide awaiting review
Purchase Creepy Crawlers Decoration
Buy 
Spooky Supplies from Izzy Hollyfizzle, who stands near the fire at the center of the Horde garrison, or next to the water fountain at the center of the Alliance garrison. Use those Spooky Supplies to purchase the Creepy Crawlers decoration from her.
Apply Decoration to Garrison
Use Creepy Crawlers in your garrison to cause Ghastly Rat, Ghost Maggot, and Spectral Spinner battle pets to spawn all across the garrison grounds.
Hunt for a Rare Ghastly Rat
Pick one spawn location and stay there — when you kill a pet without capturing it, a new one immediately respawns in the same spot and can be any of the three pets. Engage a Ghastly Rat, check its quality, and if it is not rare, forfeit the battle, kill it, and a new pet will spawn instantly in its place.
Capture Your Rare
Once you find a rare Ghastly Rat, forfeit once to heal your battle pets first, then re-engage and capture it. Note that the Ghastly Rat cannot be caged, so it is not obtainable through the Auction House.
Tips
Arachnis, a rare spawn, also appears in the garrison after applying Creepy Crawlers and has been found near the garrison herb garden, dropping the
Sack of Spectral Spiders toy on roughly a 5-minute spawn timer.
