It was my lot to help the final hours of an old black mare, who had been
abandoned to the stable's pastures for a year or two.
Large animal vets will come out to euthanize a dying animal - the local
charge is around $130.
People who have been tending horses are reluctant to put a bullet in an
animal's head - though I recall being advised (here) that the entry spot
at the intersection of lines between opposite ears and eyes, directed
towards the neck provides a humane death.
A backhoe is needed to dig an interment - people I asked have used
between 3 and six foot depths. The shallower depth went with a lime
coating - in a case of suspect west nile virus.
Brian W