After horses, a goat, a calf and other animals were viciously slashed at a Herndon park in recent months, a single question was asked repeatedly: Why would anyone do such a thing?

The mystery unfolded Monday as Fairfax County police announced charges against a Herndon teen, and the answer — every bit as bizarre as the attacks — involved a video, a black mask and a machete.

The 17-year-old was arrested over the weekend after an informant who knew the teen told police the strange story behind the slashings, which occurred in April and May at Frying Pan Farm Park and left a handful of animals seriously wounded, according to a search warrant filed in Fairfax Circuit Court. The Post generally does not name juveniles accused of crimes.

It began with a video. The informant told investigators that the teen said he had gone to a paddock near the park on April 26 to film himself riding a horse, according to the warrant. The horses belonged to the Spirit Open Equestrian Program, which teaches disabled children how to ride.

The teen donned a black mask and black clothing and climbed onto the back of a white horse, but the animal threw him, the informant told investigators, and the teen became angry. Police did not say why the teen was making the video or explain his outfit.

“[The teen] said he was mad and began to hit the horse,” according to the search warrant. “He also told the [informant] he then hit the other horses.”

Later that day, the teen took the informant to the paddock, according to the search warrant. The informant told detectives that he observed an open wound on the white horse’s thigh and blood running down its leg.

At that point, according to the search warrant, the teen made a startling admission: He had cut the horses with a 22-inch machete. The informant told detectives that the teen displayed the weapon and it was bloody. The informant also told investigators that the teen always carried the machete.

A volunteer with Spirit Equestrian discovered the three injured horses the next morning. Spirit, Tessa and Lucinda had about 10 cuts on their flanks and hindquarters. Some of the wounds were deep and required stitches. The attack made the local news and and prompted an outpouring of sympathy, but there were no clues to help solve the case.

It would take a second slashing incident to yield the tip that detectives needed.

The second attack occurred the night of May 26 or morning of May 27 at the Kidwell Barn, a popular family attraction near the horse paddocks. Henry, a calf, was slashed on his head. A mother goat named Scandal was cut, as was her unnamed baby. A chicken was also wounded. After the incident, Friends of Frying Pan Park offered a $7,500 reward.

A week later, an anonymous tipster told police that the Herndon teen had attacked the animals, according to the search warrant.

On Thursday, a detective visited the teen’s home and observed a crossbow, a spear and a pair of metal nunchaku, according to the search warrant. The teen was not home at the time.

The informant gave the account on Friday and the teen suspect was arrested on Saturday, police said. The teen has been charged with four felony counts of maiming animals, three misdemeanor counts of cruelty to animals and two counts of unlawful entry. He is scheduled for trial on July 13. His lawyer and his mother declined to comment.

The injured animals are expected to recover.

“We were worried about the possible consequences of the horses being able to serve special-needs students after the attacks, but everything is okay,” said Davorka Suvak, the founder of Spirit Equestrian. “The arrest is the last piece in the picture. It gives us a sense of peace.”