Well, Protomechs are the stage in between Battlearmor and light 'Mechs -- so they are either overgrown BA or runt 'Mechs, depending on your view.

(For those who have no idea what we're talking about, try http://www.battletecharchive.net/history/protomech.html)

I happen to have the Techreadout they first show up in....

Craig