(From the BattleTech universe, created by FASA/Whizkids/Fantasy Productions)
ProtoMechs, at 1-10 tons, are smaller than the smallest BattleMechs, which mass at least 20 tons, and larger than most Battle Armor, which masses a maximum of 1.9 tons. They were introduced by Clan Smoke Jaguar, when, facing destruction by the forces of the Inner Sphere, it needed to activate large numbers of units quickly, yet did not have the resources to produce full BattleMechs. Because the ProtoMechs required only a fraction of the resources required to produce a BattleMech, Smoke Jaguar was able to produce many more of them.
Too small to have a full cockpit, the pilot is held, curled up into a ball, in a small compartment in the center of the ProtoMech. He pilots the protomech directly through his Enhanced imaging tatoos, a technology first seen used in Elementals. Because the protomechs are so small, they are piloted neither by MechWarriors nor Elementals, rather, Aerospace pilots, who had been bred for their small size at the expense of combat skills, were used as pilots.
Unlike most Clan units, which followed rules of honor which required them to battle targets in one on one fights, rather than ganging up, Protomechs would fight in packs, making up for their small size and lack of weapons with numbers. Despite their numbers, Protomechs were not an effective weapon at stopping the Inner Sphere's Operation Bulldog, and Clan Smoke Jaguar was destroyed.
Note: this was not taken from any one source. It was done entirely from memory, though my original sources are friends and websites, including Solaris7.com and Fantasy Productions' Battletech website at ClassicBattletech.com