The skin pulls apart, and it's virtually impossible to suture. On top of that, the healing process is deceptive. Healing appears rapid. You can almost watch fresh pink tissue forming. But the new tissue keeps growing, pink, ugly and lumpy, rising far above the healthy tissue around it.
Proud flesh is false healing.
It becomes increasingly ugly when it's allowed to grow unchecked. At its worst it inhibits the horse's movement until the animal becomes lame. I know proud flesh. He lived less than two days. Today medical technology probably would have saved him.