Raid-Boss Sized Pet In Stormwind (Patreon)

This exploit will require the toy Narcissa's Mirror, note this is only obtained after a LOT of pet battling, or a lot of farming with the boon of the zookeeper enchantment. It also requires the purchase or acquisition of a Toxic Wasteling pet, and some pet biscuits from the Dalaran pet shop, which are purchased with a small number of pet charms again from pet battles.

The following steps will allow you to unleash a raid-boss sized pet in a capital city, or anywhere pretty much including instances.

Take a rogue (a class trial will do) and buy glyph of disguise, pickpocket a large mob. The biggest accessible mobs are the naga brutes in Aszuna. Summon your Toxic Wasteling pet. 

Find 5 critters-the Wasteling eats them, and everytime it does so it grows in size. Now use your pet biscuit on the wasteling. Finally use Narcissa's Mirror on the pet. 

Your pet will now be raid-bossed size-for example, it is at least twice the height of the alliance auction house. Note the pet will vanish if you mount up. 

This exploit is completely pointless but enormous fun.