When you want to get off Epona, you usually have two options. Standing still and just descend from it slowly or backhop off Epona while riding.
Both options are not optimal in most situations. Epona Slide however is the fastest option in most cases but rarely used cause it is precise and only saves a really small amount of time.
1) Ride Epona and dash to get to a decent speed
2) Hold R on the first frame, let go of it on the second and press A
3) Link will do the standard descending animation and slight forward at insane speed