Four months later, fate has proved much kinder than Withstandley ever could have dreamed.
The horse is one-third of the way through chemotherapy treatment and in remission for the lymphoma that his veterinarians found last winter, almost by accident.
Over Memorial Day weekend, she and Calvin competed in three jumping events at the Devon Horse Show.
"I love that horse," Withstandley said a few days before she rode him into the ring at one of the world's elite equestrian competitions. "He's done so much for me."
And she for him.
Although lymphoma is a common cancer in thoroughbreds, it is rare for owners to invest in extensive and expensive chemotherapy, said Amy Johnson, Calvin's primary veterinarian at the New Bolton Center, the large-animal hospital of the University of Pennsylvania School of Veterinary Medicine.
"Often, the horses have more aggressive forms of the disease," Johnson said. Usually, by the time the cancer is found, tumors have already spread to vital organs. But even in cases in which the horse might respond to treatment, owners are reluctant, either because they cannot afford the $15,000 or more it would cost, or they don't want the animal to suffer as humans do from the side effects of chemotherapy drugs.
Calvin was unusual - and lucky - in many respects.
Withstandley, a 34-year-old former Wall Street stock trader, grew up in Haverford and started riding when she was in elementary school. As a teenager, she once competed at Devon. But in college and during her years in New York City, she had missed the daily contact with horses. In 2010, when she and her husband, seeking a less frenetic life, moved back to the Philadelphia suburbs, the first thing she wanted was to buy a horse.
They had just bought a house, she was pregnant with Blair, and their budget was limited. She bought Calvin for the relatively bargain price of $5,000.
Bred and born in Florida, Calvin - whose racing name was Darksideoftheforce - had competed in 20 races and won only one. When Withstandley found him, he was in training to become a "hunter" and compete in courses that require jumping ability and agility.
"They recognized his athleticism," she said. But the horse was not happy. He seemed anxious with a rider on his back. At one point, Withstandley thought about giving up and retiring him to a farm.
With encouragement from her husband and the horse's trainer, however, she persevered.
"I gave him a month off, and when I started riding him again, cut back to only once or twice a week instead of every day," she said. She bought him a new saddle that fit him better and got him a softer bit.
She gave him a new show name, Stateside, in honor of his American roots.
Gradually, he began to trust her. They entered competitions and began to win.
"Calvin has never seen a jump that he didn't like," she said. "He jumps so hard and so high, he's never going to put on the brakes."
And while she sometimes feels that she's being carried by a wild beast, Calvin could not be gentler when he is in the stable around her and her little girl.
"That's my horse right here!" Blair said last week, as she helped groom Calvin for Devon. Wearing a pink polo shirt, flowered jeans and knee-high rubber boots, she stood on a stool so she could reach Calvin's chest, and run a curry brush over and over the same spot on his glossy mahogany coat.
The horse, towering over the child, turned his head and tenderly mouthed her shoulder with his lips.
Withstandley has never told Blair how sick Calvin was, only that their many trips to the doctor are needed to keep him healthy.
And because the horse has never shown any signs of illness, or suffered any visible effects of chemotherapy other than some thinning of his tail, it has been relatively easy to keep the conversation light.
Back in January, she had noticed some swelling around his throat. At first, the doctors thought it was "strangles," a common bacterial infection. During an ultrasound to examine him more thoroughly, Calvin was given a sedative. The drugs made his penis, which is normally retracted, relax. That was when Johnson noticed a few nodules on the sheath. Within a few days, more bumps had appeared on his haunches.
"It was advancing quickly," Johnson said. But after the initial biopsy, more tests were done that showed that Calvin had subcutaneous lymphoma, a form of the disease more treatable than many others.
"They gave me three options," Withstandley recalled. "I could do nothing, treat him with steroids, or give him the chemotherapy."
Her health insurance for Calvin allowed $7,500 a year for major medical treatments, which was used up during the first three weeks. The subsequent nine months of chemotherapy would cost about $15,000.
She and her husband were ready to sell off some of their savings, but Withstandley's parents pitched in to help.
In Johnson's seven years at New Bolton, Calvin was the first patient to be treated with chemotherapy for lymphoma. So she consulted with Penn Vet grad Angela E. Frimberger, who now works in Australia with a group of veterinary oncologists.
Frimberger advised Johnson about which chemotherapy drugs to use as well as the sequence and duration. The treatment is roughly the same as the one used to treat humans with the disease, but the doses - relative to the horse's weight - are reduced so that the animal does not suffer any side effects.
"We want to preserve quality of life as much as possible," said Johnson. "People are more tolerant because they understand what's going on."
Although Calvin's tumors disappeared after only two treatments and he appears to be in excellent health, it is hard to know whether the proportionately lower doses will be less effective over the long term.
"There's very little research out there or long-term studies," Frimberger wrote by e-mail.
Meanwhile, the Devon Horse Show was timed perfectly - between chemo treatments.
Calvin received his most recent dose in April. For several days afterward, because his white cells are reduced and he is more vulnerable to infection, he cannot be around other horses, Johnson explained. At this point, however, he is fine to compete. His next treatment is scheduled for this week.
"Blair says that when she gets older, she'll jump Calvin at Devon," Withstandley said. "I hope that happens."