You're in luck. Epona will have a chance to respawn as long as you haven't registered her yet. I lost my Epona on a cliff she couldn't get up or down from. Later on she spawned again for me. Took about 20 days of Link scans though. It's a rare chance. You'll find her again eventually. If you don't believe me, there are other people online who have been in similar conundrums like the two of us. More than one person freaked out after spawning her on the Great Plateau, for example. A lot of people gave up trying to get her back, but they shouldn't have reset the game. Nothing has been lost but one day of amiibo scan. By the way it's not a glitch. This is a feature, so that Epona isn't lost forever. Once she's logged with the Stable system, she won't spawn from the amiibo anymore, so take care of her when you do get her back! I recommend always using the TP Link amiibo figurines in front of a stable for safety purposes.