We have been fortunate to never run into foxtail ulcers of the mouth prior to two of our latest intakes.
Foxtail is not something a horse will generally eat when growing in a field, unless they have no option. When mixed in hay, the horse will inadvertently (or who knows, maybe happily) consume this hateful plant, and the heads embed in their mouths, and it can require tedious extraction by a vet.
Luna has one giant ulcer in her lip that had clearly been there for a long time given how hardened the tissue was around the ulcer and how deep it had become. Further, it was very painful. It took over an hour of pulling out bits of foxtail to remove it all.
A few weeks later, she is finally healing well.
Ripley had a smaller ulcer that healed with flushing.
Unless we checked mouths like we do, this would have been overlooked a long time. So this is a note to head out if there is any question and periodically check the mouth, lips and cheeks of your horse for injury. You never know what you will find.
Foxtail can do even more damage than you see in the photos of Luna below. Please share and be diligent about searching your hay for this rotten plant.