Angus Lake is charged with four counts of aggravated driving under the influence.
Another woman was also struck. Heather France is hospitalized in fair condition.
Eustis had been dating Lake, a harness driver, for about a month.
The accident took place around 5 a.m. Monday morning at Balmoral Park Racetrack in south suburban Crete.
Prosecutors said Tuesday that Lake, who was convicted in 2005 in Michigan of driving while impaired, admitted to police that he had been drinking before the accident and that his blood alcohol level was nearly two times the legal limit.
Eustis's father told ABC7 that his daughter had been dating Lake for about a month prior to the accident. He is finding it hard to grasp that his daughter is gone.
"She shouldn't be gone," said Charles Eustis. "There's no reason for it."
Eustis said all his daughter wanted to do from the time she was 3 is ride horses. As she got older, she worked for her father, who trains horses at Balmoral Park Racetrack.
"She worked for me in taking care of all the horses," said Eustis. "She would do them up during the day and get them ready to go out when we train."
During her years working at Balmoral, she grew close with Heather France and Angus Lake, a horse driver at the park. After a few year of friendship, her father says Eustis, who was 25, and Lake, 41, began dating a month ago.
Investigators say the three -- Eustis, France and Lake -- were together Monday morning when the tragic accident happened. They say the three were drinking on the grounds of the park. Then Eustis and France jumped on a horse, riding bareback. After a while, Lake jumped in his truck to look for them and ended up side-swiping the horse.
Lake was allegedly speeding when he hit the horse.
Eustis died of head trauma and the other rider, France, suffered a broken leg. The accident happened on the backstretch of the park.
Officials say they will review security procedures.
"We will continue to participate with the state police in their investigation, and we'll review our policies and procedures at Balmoral to make sure that nothing we could have done that could have prevented this unfortunate situation," said Balmoral Park general manager Michael Belmonte.
Michelle Eustis was the oldest of four children and a mother to a 6-year-old girl who also loves to ride horses.
France and the horse are expected to recover from their injuries.