While no-one wants this situation, it does occur unfortunately. Most mares that die with young foals do so very early on in the young foal's life - ie at or within a few days of foaling.
When a deep wound in a horse happens, it heals by producing granulation tissue to fill up the hole created by the wound. When there is too much granulation tissue produced, it bulges out from the hole producing "Proud flesh". This looks like a flesh cauliflower, and is common in wounds in the lower leg area.
This condition causes lameness in horses, consistently in the front legs. Commonly it will start with a slow onset of lameness in front, sometimes in one, then the other front leg, eventually to both at the same time. The horse develops a short, choppy stride seen easiest at the trot.
Horses, like all animals, have specific requirements for minerals and vitamins for their well-being. These are usually supplied in their diet; or the horse manufactures them from products in the diet.
In most cases of misadventure the laceration will be on a limb. In the mildest cases hair loss and bruising may be all that occurs and you may not need to call your vet. Other cases of injuries may involve loss of large areas of skin, deeper injuries perhaps with exposure of bone or the horse may be showing pain.