Guess i'll answer my own questions.
Have you ever used Haxorus before, competitively or in-game? If so, when?
Yes, I have. I used it at least once in Black a long time ago, and I used one in my first ever playthrough of X when I first got the game back in October 2013. And I also used one in my White 2 Challenge Mode Wedlocke back last spring, but it came super late in the game and only got real action against Grimsley and Iris (i'll get to the latter in a bit).
Do you like Haxorus' design?
I would have to say yes, I most certainly do. Those axes at the top of its head give it a fearsome demeanor to it even though it's meant to be described as a "gentle giant".
Do you like its Dragon-typing? If not, would you give it another type alongside it?
Yeah, I think it fits decently. Although it looks like a Pokémon that would have been neat if it had a Steel-type to it, because those axe jaws and its design really supplement that decently if you ask me.
If you were to give it a Mega Evolution, how would you go about it?
I think i'd up its Attack a little, and the defenses and Speed. Maybe something like +20/+20/+0/+30/+30 (so 76/167/110/60/100/127). And give it a Bug-type, too, because that's another type that it could also make perfect sense with. And the Tough Claws ability I guess? Wonder if that'd be too generic. Oh, and let it have Megahorn. That'd be nice.
What's your overall opinion on Haxorus?
Although it was up there as another dragon that got hit pretty hard with the introduction of the Fairy-types in Generation VI, it's still a fun Pokémon to use, if a super strong one at that.
Bonus question: Did you ever have a difficult time with Iris' Haxorus in her Champion battle in B2W2?
Well the very first time I played White 2 I remember it providing somewhat of a challenge since it could SE my entire team except Samurott, who I had to keep alive to manage to beat it. And in the many challenges i've done, i've definitely had to take it hard. To name one in particular, I remember one time when I did a No Evolutions challenge way back in 2013, my only Pokémon remaining was Elgyem and when she came out, I had to revive something and she used Dragon Dance, and from there on out it was like almost ten minutes of trying to stall it out with Revives and hoping my Elekid would get Static on her, which it never did. It eventually killed itself from multiple hits of confusion damage.
And with another go on Black 2 that I had in August, I remember getting greedy with my Escavalier and setting up +4 Defense with Iron Defense when she had only that at about a third of her health, and Archeops, left. I just had to survive the next hit...but she scored a critical hit, which normally would be impossible since I had Shell Armor, but she had Mold Breaker which says screw you to that, and I failed. The second try (well, third because I restarted after my Scolipede missed against her Hydreigon then she OHKO'ed with Fire Blast), she OHKO'ed my Lucario with Earthquake, and my Espeon got in a Reflect and did over half her health with Psychic before she 2HKO'ed me with Outrage. After that, my Drapion managed to finish it when she hurt herself in confusion.
If I kept talking about other times i've dealt with it i'd be here a while so I think i'll stop there.
Anything else you want to mention about it?
Not really.
And there you go.