I know it's not an attack, but the whole thing is similar in concept. That's what I was getting at. You'd be getting an unusual Pokemon that you can only receive through an in-game trade. That's what BR25 was suggesting: giving an NPC a Thunder Tentacruel and allowing them to give it to you via an in-game trade.
I mean, personally, I'm more in favor of just having the Move Tutor concept, but that's not to say there couldn't be some event in the game where you get an unusual Pokemon via in-game trade. It might be useful somewhere.
Also, neat banner, Miksy! I'll add it to my sig.