Zander

Hydreigon was very usable in gen 6 OU as I have used it multiple times in a team that had barely one pokemon that qualifies for OU. Nowadays all the Pokémon who couldn't keep up in OU have been shifted to UU which is why Hydreigon is still so effective there. In the current OU Hydreigon gets bodied because of the Pokémon who can rush kill it or stall it out. Just like how Dragonite is nothing without multiscale, Hydreigon would benefit a lot from wonder guard as it could withstand some abuse.

As for the lore part of it, Hydreigon is literally the worst adaptation of a lore I have seen. The only thing that connects Hydreigon to the Hydra is it's 3 heads. Following the lore, it should have been a Poison/Water type and actually be able to learn a good Poison type move like sludge bomb/wave. Also the hydra could not levitate at all so levitate is literally the most random ability ever. I feel like Hydreigon was only given dragon/dark to give more meaning to Ghetsis, who is the first ever trainer to use it. The hydra is supposed to grow two heads for each one cut. The ONLY way to kill it is to either cut off all it's heads at once or singe each cut off head with it's own poison so that it doesn't grow. If it were to be compared to a Pokémon ability then I can't think of anything other than wonder guard because it only allows super effective moves to do damage while making the Pokémon immune to the rest of them. 

I don't think you guys would understand what I'm trying to say here but basically Hydreigon has OU potential by a simple change of ability and no it won't be broken due to it's common weakness generally okoing it.

I think that the reason people such as myself find this so entirely laughable is 1. Hydreigon isn't a pokemon that needs a buff. Sure, its not OU, but so are 90% of other pokemon. Why does it matter if you can't use Hydreigon well in OU, that's natural as the Meta is largely focused around Pokemon that Hydreigon isn't even good at fighting. Hydreigon is a perfectly servicable Pokemon on paper, it's just anti-meta. That's basically the entirety of UU nowadays.

2. Why WonderGuard? Even if we were to believe that "Yes! This Pokemon needs substantial buffs in order to be good. Why would you choose WonderGuard? That is one of the most uninspired ideas for a buff ever. Any Pokemon would be better with Wonderguard. Hydreigon already has 2 immunities, let's just give him 11 more! That would make it so Hydreigon literally walls most Pokemon in the tier. If you didn't have a way to OHKO Hydreigon, You'd just flat out lose. Not to mention that Hydreigon can actually survive SE attacks unlike Shedinja, and has access to healing. It's evident that very little thought about how a WonderGuard Hydreigon would actually function in the current OU Meta, so the whole idea seems incredibly banal

3. Powercreep happens, Hydreigon isn't even the earliest or most tragic example. Remember when Tauros of all people ran the metagame? Remember when Flygon was OU? Suicune, Gengar, Celebi, and Scizor all used to be OU staples. They're not anymore, but nobody thinks they need Wonderguard to function well. If you really like Hydreigon, play UU or something, he's really good there!