If you are implying that Heracross doesn't learn Megahorn until lv.53, this is also a move that can be passed on Egg if a Male parent is an Heracross that knows it.
Basically, an Egg Heracross can legitimately learn Megahorn at lv.5, while a Wild one cannot until 53.
Sources:
Since just one move has passed from Male parent to Egg, I've failed a few tries until I used as a parent a Male that only knew Megahorn.
It's different, for example, for Agility Metagross at lv.50, because Metagross learns Agility at lv.66 and sadly this move is not in Beldum learnset, so it cannot be passed through breeding and then this set is illegal.
It may not be that immediate and I don't know if submenceisop has done these passages (that are really easy, for sure way easier than RNG a flawless Hera), and since his Hera is in a Poké Ball looking at the video (other Balls of course are not possible if breeded), his set is potentially legal.
We have on leaderboards players that admitted to use fake Pokémons with legal sets, so I don't think it should be an issue.
Thanks, I actually did it myself since someone else brought this to my attention and didn't wanted to throw anything without evidence of this beforehand. I was completely unaware that this mechanic existed (I guess I got too used to the modern mechanics of re-learnable moves). Either way, this is good to know and makes it possible to use Megahorn Heracross at Lv.50 assuming you're ok with that awful accuracy.