It is hella obscure, I can't remember the exact phrasing, it comes from a 20th or 21st century book. It comes from an English language novel (most likely American), it's short. Pretty sure it's a guy addressing his horse, using the horse's name and saying something like "won't ride any more" or "can't carry that any more".
The horse's name is pretty simple, and it may even be a relatively common name (perhaps Spanish).
It's not a popular phrase - originally I learned it in Russian and then was surprised to find out it was actually copied from that English-language novel.
Anyway if anyone is able to find it I'll buy you a $20 game or something of equal value, it is that aggravating to me I feel like I am losing my mind especially when I can normally find things just fine online and now the results I'm getting have nothing to do with my search.
RS3: UltimaSuende - CE Thread Zone
https://letterboxd.com/BMovieBro/