That's not a very good excuse. They could've made Garchamp and Tyranitar the counterparts and had Aggronite in both games. I think their choices in counterparts for XY were based more on design rather than actual stats and such. So a Mega Mightyena would've worked due to a similar body.
I'm actually in agreement with Zekrom in saying that you guys are seeing counterparts where there aren't necessarily any. If they were really diametrically opposed counterparts, the entirety of one would be exclusive to one game while the entirety of the other would be exclusive to the other. Instead, we have this system where one element of each Mega Pokémon is in one game and another is in the other. Example, Houndour are only found in X, while Houndoominite is in Y.
So...which game does Mega Houndoom correspond to? X or Y? You cant get a Mega without having its respective Mega Stone, but you obviously can't get it without the Pokémon itself. So, therefore, the correct answer would be "neither". You need collaboration between the two games in order to get a Mega Houndoom, which can be in either X or Y.
Before, with actual counterparts, you needed collaboration with the other versions just to either see or capture the other Pokémon altogether. Think Lotad vs. Seedot in Sapphire and Ruby, respectively.
The system of splitting up Pokémon and Mega stone between one version and another is not completely analogous to the counterpart system we had back in the day because you are just as likely to have Mega Houndoom/Manectric or Mega Tyranitar/Aggron in X as you are in Y.
HOWEVER, if we were to classify these Mega Pokémon as counterparts based, like you say, on appearance rather than stats, we could make the argument that Mightyena still wouldn't be a good counterpart for Houndoom. There's a fine line between counterpart and clone. Gamefreak probably thought that having
two Dark type, hound/dog-like Pokémon both getting Mega evolutions might have been looked at as a little too redundant. Manectric allows for a a dog-like Pokémon, but of a different type.
However, we are in agreement that Mightyena should get a regular evolution instead of a Mega. Personally, I'm hoping for a werewolf-based Pokémon (hopefully named "Wereyena") that would gain a secondary typing of either Fighting (because for some reason, I correlate werewolves with street fighters) or...Fairy (because of werewolves' actual correlation with the Moon, and the link that appears to be between the Moon and Fairy types in Pokémon world). Or, you know, just a pure Dark type would be cool, too.
Megas are cool and all, but I just don't want
everything to be engulfed by Megas. Let's save some room for tacking on regular evolutions and creating new stones...