Foal scours, a sort of diarrhea of foals that generally occurs near the mare's first heat after foaling (referred to as the foal heat),
can be a tenative time for foal and owner.
Altho it may not look "normal", almost every foal goes thru a bout of the scours.
It can be helped with the following products: probiotics (live-active-culture yogurt as an alternative) to establish friendly bacteria, Pepto Bismo or Kaopectate, Endotox (a gel available from the vet).
Worm the mare with ivermectin just prior to foaling or on the day of foaling. Some research shows
that scours could be a result of worms from the mare.
Watch for dehydration of the foal; call the vet if necessary.
From Alabama Cooperative Extension:
In the past, horse breeders thought hormonal changes in the mare's milk during foal heat caused diarrhea in the foal. Recent research has implicated an internal parasite (Strongyloides westeri) as the true cause of foal heat scours. This parasite is transmitted from the dam to the foal through the mammary gland. Foals begin to shed eggs in their feces 10 to 14 days after birth, resulting in scours that coincidentally occur with foal heat in the mare. If the foal is alert and nursing regularly, mild foal heat scours usually do not harm it. However if the foal stops nursing and becomes weak or dehydrated, consult your veterinarian immediately.