Founder aka Laminitis
It's grass season which also means it's founder season. It's a mysterious disease that effects horses who typically consumer too much lush grass, although there are several types of founder. Fat ponies, older horses, insulin-resistant or "IR" horses are all prone to founder.
Founder or Laminitis is extremely painful for afflicted horses. Some early signs might include heat in the hooves, spreading their front and back legs to "park out" as a way to help ease their discomfort, lameness at the walk, or shifting from one foot to another frequently. If you suspect that your horse is foundering, call the vet immediately, pack their feet in ice. Time is a huge factor in helping to stop the destruction in your horse's feet.
You may have to monitor your horse's grass intake in the spring and fall when the grass is lush. Lush grass is not your horse's friend. Grazing muzzles, limiting grass turnout, frequent exercise, and a proper diet can help to prevent this terrible disease.