The Grey Cat Tail can be found on the second floor of the Clothing Store and purchased for 15 Bucks. Purchasing this hat will allow you to wear this hat only in Adopt Me!, unless you buy this item from the Roblox Catalog.
The description of this item on the Roblox Catalog reads
"Celebrating World Cat Day in style with your very own cat tail. This tail goes on the back of your torso (as is customary for tails) and not your head."