Would it be okay if I bumped this...?
Anyway, I'll give my thoughts on the legendary musketeers (Cobalon, Terrakion, Virizion, Keldeo) in competitive terms. Aesthetically, I was surprised when I learned that they were quadrupedal Fighting types. It took me a while to figure out how they were able to use their signature Sacred Sword without having to stand on 2 legs (I know now that they perform the move using their horns).
Cobalon: It's got the same typing as Lucario, but its offenses are weaker. It can take physical hits better than Lucario can, but its Special Defense is a bit exposed, but it can be fixed with Calm Mind.
Terrakion: The only musketeer to have a unique type combination among the four (Cobalon has Lucario's typing, Virizion has Breloom's typing and Keldeo has Poliwrath's typing). Rock/Fighting provides good coverage. Looking at Terrakion's stats and movepool, it does one thing very well: physical sweeping. It can rip holes through most opponents, and can take hits well (though it isn't as bulky as Cobalon or Virizion in the physical and special departments, respectively).
Virizion: Like Breloom, Virizion is Grass/Fighting, giving it a natural resistance to the QuakeEdge combination. Its Defense is low, unfortunately, and Grass/Fighting is resisted by Flying types (who are taken care of by Stone Edge). What I do like about it, though, is its high Special Defense, which is the highest among Grass and Fighting types; that can be amplified further by Calm Mind, but its low Defense is still a problem.
Keldeo: When I first saw it, I thought it looked like some sort of donkey/pony. After seeing it in battle, I developed a newfound respect for it. That's because it learns Sword of Mystery, which works in the same way as Psycho Shock. Its Attack and Special Attack are the reverse of Terrakion's. At a glance, Heart of Justice seems like a waste on Keldeo due to its 72 Attack. But with Swords Dance, that stat can be pushed further.
