Crashin' Thrashin' Robot
From Crashin' Thrashin' Robot. Well, it's fun to make them, but the "non-combat pet that fights other non-combat pets" is wrong.
The 
Crashin' Thrashin' Robot is an Engineering-crafted toy that deploys a tiny mechanical robot you can pit against a friend's robot for chaotic, attention-grabbing battles.
Guide awaiting review
Gather the Raw Materials
Collect the following materials to craft one robot: 1 
Primal Earth, 8 
Adamantite Ore, 14 
Fel Iron Ore, and either 1 
Gold Ore or 1 
Iron Ore if you have the Transmute: Iron to Gold ability.
Craft and Deploy the Robot
Once crafted, use the item to place your robot — note it is extremely small (roughly the size of a toe) and does not follow you around. The item has a 3-minute cooldown, after which the robot breaks apart and you can place a new one, while the item itself remains in your inventory.
Stage a Robot Battle
For the full experience, find a friend who also has a Crashin' Thrashin' Robot and deploy both in a public area — the two robots will fire machine guns at each other, typically destroying each other simultaneously, and produce a loud crash sound on death that tends to startle nearby players.
Tips
These robots do not fight Clockwork Rocket Bots, but as of 6.0 they will attempt to fight Blingtron 4000 and Blingtron 5000 — be aware that Blingtron one-shots them since they are level 1 mechanicals.
