I thought I would have this opponent in one of my challenger lists!? Some of my pets already fought against him, but I can't find him anymore. I also do not remember, how I unlocked him. Is it possible to unlock him again?

It's an old plot challenger. The only way to fight it now is in battle quests (BQs). It can randomly be picked as one of the Very Hard difficulty challengers while BQing.

Thank you. I did not remember anymore, where those battles, my pets fought, came from.