I highly doubt it came from a first nations culture in that area, it's either an artwork placed there by the artist, or someone's attempt to fool you. Large sculptures (I'm leaving out petroglyphs and trail markers) from native American cultures are typically located in a settled area with a large population base, think Mexico City, Tikal, Palenque,Totem poles of the Pacific northwest, and Adena & Mound cultures. Even Worldwide sites are the same way, if it's a sculpture, monument, or bust, it's where a major city or village was. You just don't find something of significance in a random spot in Hill Country without finding a village, you just don't. Is it cool...yes...but it's not real (artifact)
Edit: Most of the tribes in that area did not have the time and resources to dedicate to sculpture IMO, water, food, shelter, and family would have been top tier priorities.