A steam tonk is a controllable toy tank found at the Darkmoon Faire.
About
The new (as of Patch 1.10) Dwarven-made Steam Tonks of the Darkmoon Faire are a huge success. Players can get to a control panel anywhere on the faire, and control a little, steam tank like tonk. With this Tonk, you can only attack other Tonks, not players. In short, Tonks are a cross between the safety of a duel and the multiplayer madness of a battleground. Note: They dismiss your pet when used.
Players with an Engineering skill of 275 can obtain the schematic for a Steam Tonk Controller from the Darkmoon Faire. The schematic costs forty Darkmoon Faire Prize Tickets. A Steam Tonk Controller can be used by anyone - it is not bound to engineers only. A Steam Tonk Controller has fifty charges and requires 2x Thorium Widgets, 1x 
[Mithril Casing], and 1x 
[Gold Power Core]. The Steam Tonk Controller will summon a Steam Tonk pet that remains active for five minutes or until the Steam Tonk is destroyed. Steam Tonks can be destroyed by other Steam Tonks and by going outside of the range of the Steam Tonk Controller. A summoned Steam Tonk can not engage a hostile NPC in battle. While steam tonks cannot be summoned within the major cities, they have been summoned on sea-going vessels and inside instances.
When
Tonks are available for use during the Darkmoon Faire at Thunder Bluff or Goldshire during their respective months.
How
Tonks are controlled using your movement keys (default or custom) and using the extra spell bar, similar to hunters pets or warlocks minions. Each Tonk also has a special ability like 
[Mana Shield], Flamethrower, etcetera. They can also jump.