Stackable Stag
Sold by Sally Fizzlefury. Most likely a reference to stag stacking, a hilarious bug that afflicted druids who had Glyph of the Stag for some time after 5.0.4.
The Stackable Stag is a toy sold by Sally Fizzlefury in the Valley of the Four Winds, serving as a tribute to the infamous druid stag-stacking bug from patch 5.0.4.
Guide awaiting review
Find Sally Fizzlefury
Head to Nesingwary's Safari in the Valley of the Four Winds and find Sally Fizzlefury at 16, 83. She is the engineering trainer who sells the Stackable Stag.
Purchase the Toy
Buy the Stackable Stag from Sally Fizzlefury for 
1000 Gold (before any applicable discounts). The item is not consumable and not marked unique, so you can purchase multiple copies, though they all share the same cooldown.
Tips
The toy has a 1-minute cooldown, and each summoned stag lasts 2 minutes, meaning a solo player can stack a maximum of 2 stags at a time. Each additional friend with the toy can add 2 more stags to the stack.
Take turns summoning stags rather than summoning simultaneously — if a new stag is summoned while the stack is still forming or a stag is despawning, it will start a separate stack instead of joining the existing one.
The stag spawns next to your current target within a 40-yard range, and it will draw aggro from nearby enemies in instances unless another stag is already present for it to stack onto. It can also be used while stealthed without breaking stealth.
