I was thinking about the Buffalo, and what could be there semiroyals. Bear with me here.
Plains: Stone bones/teeth/horns/ect? No idea how to approch this one, considering the only earth-spirit I can remember from native american myths was a hairy savage dwarf-like beast.
Wood: "Springy" form? Wooden "skin"? Honastly, there must be more to the myths than what I know, but I am torn about what to do here. On one hand, we can use Little Strongheart as a modal for a lighter, more flexable form of buffalo, with a the ability to not take as much damage from blunt attacks. On the other hand, the whole "wooden skin" thing fits the MO of a powerful shamen-like buffalo.
Eagle: This is where the fun starts. Two ideas for this one. One is that they carry a contact syndrome on there hooves that causes blisters, extreme pain, long term crippling of movement and the like to simulate a "Thunderbirds" attack. If it can shoot it in glob form, that would be preferable. On the other hand, the wings could be replaced with a semi-solid substance called Wind, as so that they could never be damaged by the occasinal lucky sword-swipe. Naturaly, they would be faster than normal.