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Just so, which is better Haxorus or Hydreigon?

I would say hydreigon is the better pokemon. Because it can learn many more moves than Haxorus. Also, hydreigon's stats are better - they are all round great while haxorus lacks in all round stats, although its attack is better than its competitor. Hydreigon lacks swords dance/dragon dance which are great for Haxorus.

Subsequently, question is, is Haxorus better than Garchomp? Salamence has the most well rounded offensive stats, garchomp has the best coverage for fairies out of the group (poison jab), and Haxorus can destroy things with his high attack. So it comes down to this: do you want coverage for fairies, or do you want a slightly more potent offensive force.

Regarding this, is Haxorus good competitively?

But the Nature and Ability are great. If you plan on doing competitive later, you can use that one to breed ones with better IVs. Give it Earthquake to make the best use of its Mold Breaker Ability, to hit Levitate pokemon and OHKO Sturdy pokemon. Haxorus is a great pokemon to have, make good use of it.

Is Haxorus legendary?

Haxorus, when at Level 100, only has a BST (base stat total) of 540. 60 short to be considered a Pseudo-Legendary Pokémon. It is, however, a really great Pokémon, even if it isn't a Pseudo-Legendary, it still has better stats than some of the lesser legendaries.