My daughter had a foal born with entropion and took her to the vet. I don't remember, but I think they went the suture route to avoid the possibility of ulcerating the eye. It depends on the severity and a vet should make the diagnosis and course of action.
We had one--I just rolled the eyelid out a couple of different times and then it was fine. When I rolled the lid out I also dried the lashes off with a soft cloth--with the lashes wet they just wanted to roll back against the eye again. Dry they were more inclined to stay away from the eye.
Entropion has an hereditary component, so pay attention to future breedings.
I have very good success with rolling out the lid and pinching it to create a swelling that will keep it out. Once is usually enough, but it's cheap and easy so if I do it 2 or 3 days in a row no big deal.