1. To fit a saddle onto an animal, especially a horse, in order to ride it. In this usage, a noun or pronoun can be used between "saddle" and "up." Norm, go saddle up the horses for our guests, please. We finished saddling the mules up and got ready for our descent into the Grand Canyon.

2. To sit in an animal's saddle in order to ride it. There's a storm coming—we'd better saddle up and head back to the ranch.