After searching for his stolen horse, Blue rides back to the ranch to get a fresh mount, saying that Soapy, the horse he's riding, is exhausted after three days of searching. He leaves a few moments later on a big chestnut horse with Buck, Sam and the rest of the search party. In the next scene, though, when he's chasing Chatto, he's back on Soapy again.
When Blue and John are discussing the stallion, they look over at him in the corral, and he has no halter on. In the next scene, he does.