Mecha Jr. is the midboss of the Lava Pit in Mario + Rabbids Kingdom Battle. As its name suggests, it is a mecha operated by Bowser Jr., resembling a large robotic Rabbid. It has long, relatively thin limbs supporting its cylindrical body; the feet have rocket boosters and the arms end in pincers. It is armed with a massive cow-like hammer. It also has a compartment in its chest, used to store Spawny and deploy mobile bombs during the battle.
Mecha Jr.'s behavior is similar to that of a Smasher; it charges at the heroes in an attempt to smash them with its hammer, which can inflict Burn on contact in addition to massive damage. Like a Smasher, it moves prematurely towards its attacker when attacked at range; unlike a Smasher, it is immune to movement-based attacks such as Dashes and Team Jumps. It has two additional abilities: a variation of Magnet Dance to draw characters towards it, and mobile bombs (similar to Luigi and Rabbid Peach's Sentry weapons) that also head towards the heroes; if set off, they inflict high damage with a chance of Push effects. A pair of Lava Valkyries are present to support Mecha Jr.
When Mecha Jr. is defeated, it malfunctions and bops itself on the "head" with its own hammer. Bowser Jr. narrowly escapes his mech's destruction, but drops Spawny in the process as he flees from the heroes. Luckily, Yoshi rescues Spawny, and both join the group.