Possibly a reference to Johto's beast trio or possibly just showing off they that they know about American history. They have referenced nuclear bombs with Victini, our country's symbol with Braviary and now this to reference the history of Texas and the Mayans/Aztecs/Incas.
Doubt its a reference to Johto's trio seeing as only Landorus is a Beast. Tornadus is a Bird, which would be Kanto, and Thundurus is Dragon like which has no region smaller trio and can't match Sinnoh as they were 4th Dimension bodies.
Well I must say I actually like the new forms for Tornadus and Landorus after seeing better pictures of them. I didn't like them at first just from the cover. I'm thinking the new Landorus form is my favorite of the three. As far as Thundurus goes, I have no idea how I feel. Part of me wants to like it's new form along with the other two, but then part of me wants to hate the design of it.