Bots that have no human masters are tired of seeing helpless trainbots getting pulverized and decide to step in.
They are trainbots that are near or similar in level as your bot when you meet(unlock) them and are equipped like regular bots from head to toe with unique gear or a mix of unique and non-unique gear.
Designed to add some flavor into the career of a leveling bot's life. Tough enough to provide a challenge but not demolish the player. Possibly forcing the player to change 1-2 of their gear set up in order to beat them (have more con, a little boost in str/dex to get the next weop, etc.)
Unlock by level and amount of trains. Eg. 1st unique trainbot unlocked at level 50 and at 200 trains(can't remember how many trains to get to 50, just an example). Number of trains are a requirement so it is something for leveling bots only. Of course fighters can get in on this if they devote the amount of trains to unlock them. Amount of trains has to be high enough that this feature becomes almost exclusive to leveling bots (or at least they will reap the most benefit from it) but low enough that leveling bots won't over level the level requirement.
Reward could be a boost in exp (say 30 trains worth of exp) that gets lower as you level further away from the unique trainbot. After beating the unique, it becomes a display trophy stating when did you beat the bot and possibly how many attempts you had on it.