Could have been elk. Venison is darker than beef.
Ostrich is a LOT like beef as well. Someone could eat that and not know it was not beef.
I told the relative at the time that I didn’t think it was beef and since selling elk or venison is illegal in the US he was shocked. He had bragged to me on the way to the butcher shop that New Mexico beef was better than Texas beef. I never heard anymore about it after that.