Two things clearly standing out are:
1) lack of Protect together with Hydreigon having Surf and Haxorus potentially locking into Earthquake (and Excadrill and Terrakion being weak to it). More on Surf later, but yeah, you should have Protect on all your non-Choice pokemon. Maybe Whimsicott can get by without Protect if you really feel all 4 moves are that useful.
2) Surf with no immunity isn't good, but what's even worse is Normal Gem unSTAB Hyper Voice for Hydreigon? I understand Hyper Voice hits both targets, but it really isn't that useful, hitting nothing for super effective and resisted by a few fairly common VGC pokemon (Chandelure, Jellicent, Terrakion, Bisharp, etc.)... Surf is ok since at least it's resisted by Haxorus and Whimsicott, but Surf is primarily used only on Hydreigon because of Jellicent (Water Absorb) and Musharna (Telepathy). STAB Dark Pulse covers Psychics and ghosts better. That said. Dark Pulse IS an egg move, so if you don't want to breed again, sure, your choice of course. I'd still get rid of Hyper voice for Protect and replace item with White Herb (for Draco Meteor) or something like that.
EDIT: Also, yes Terrakion definitely needs to be Jolly. And to be honest, I don't know what's going on with Thundurus. All I can really say about that is I'd probably want Thunderbolt somewhere there regardless of whether or not I'm keeping Discharge.