There is no trio master for the musketeers, Keldeo is a special extra member. If anything he is the adoptee rather than any kind of master. Its role can be compared to Regigigas for golems or Rayquaza for the RS mascots or Landorus for the Kami (except that all those were actually still superior in their obvious group), but it cannot be compared with Ho-oh and Lugia as the beast and birds guardians (who the trio master term was actually inspired by), which is the kind of role I was suggesting for Xerneas to the musketeers as a very equal quartet, as unlikely as that may be.
Gamefreak doesnt share the poorly defined fanon concept of "trio masters". In fact its like every generation they add a slightly more deviating kind of "extra member", which, after a few more, will probably result in new terms being coined for groupings, or a more generalized term that does not limit to a specific number (trio), or a superior role (master). IMO its already outdated now, but what can I do lol.
Anyhow back to Xerneas.
It sure does look different (in that it is not "just another musketeer" like Keldeo), but so do good old Ho-oh and Lugia to their respective groups. With the 4 sets of antlers and a deep association with nature (I am guessing) it would actually have SOME conceptual connection to them.
Also, the musketeers originate from France, although this likely doesn't mean a thing.
What fascinates me about the idea though, is that it would imply a trio+extra with a connecton to "Yveltal".. would they be some kind of incarnations of bad things? Perhaps as polar opposites to musketeers, wield secondary Dark types? Would they be airborne? If yes, some kind of Wyverns or perhaps a second set of legendary birds even?