The All Terrain Old Prototype, or AT-OP... okay, that's more of a backronym it acquired once the AT-TE was invented... was the mechanical ancestor of the entire "AT" walker family. The AT-OP was a goofy-looking walker unit that stood three-quarters of a meter high. It had no weapons and was mostly used to carry extra gear for Republic troops.
It walked fairly well and could even recover its footing when somebody kicked it... which meant that people ended up kicking it a lot, just to see what happened. It could carry a wounded soldier off the battlefield, or give the commanding officer's spoiled daughter a "horsey ride." It wasn't a droid and couldn't think or speak... but in a pinch, it could let Uncle Martin know there was a clone trooper trapped in the well.
Kinda looks like a big dog, doesn't it?