In 2016, Tracy Jennings received shocking news: A lifelong friend, a woman who had a farm with animals great and small, had died suddenly in an accident.

A circle of grieving friends hastily arranged new homes for the woman’s beloved animals, including three older horses. But just two weeks after her death, her family had the horses euthanized.

“It was very sad, a real kick in the head,” Ms. Jennings said. “But we had no legal rights, and she had nothing in writing.”

Ms. Jennings, 66, a semiretired landscape designer, lives on a 10-acre farm in Buffalo, Minn., and has her own menagerie: five cats, a golden retriever, two horses and a donkey named Niles. They would not meet a similar fate, she decided.

“There’s nothing sadder than seeing pets wind up at a shelter because the older person who cared for them has died,” she said.

She had a lawyer draw up a pet trust, a legal document specifying who will care for her animals when she dies or becomes unable to do so herself. Her trust provides money to cover veterinary bills, food, grooming, boarding and unexpected expenses. It specifies which vet to use. Attached guidelines impart care instructions, including particular foods and animals’ “personality characteristics.”