
The pictures widely shared on social media show the riders using their reins against the migrants and pushing them back towards the Rio Grande in Texas.
Homeland Security Secretary Alejandro Mayorkas says his department will investigate reports of alleged abuse.
He said the officers were trying to manage the migrants crossing the river.
Some 13,000 mainly Haitian migrants have gathered in a makeshift camp under a bridge connecting Del Rio to Mexico’s Ciudad Acuña on the US-Mexico border.
White House spokeswoman Jen Psaki said the footage was horrible to watch.
Some social media users said the pictures were reminiscent of the violence seen in slavery times in the US.
The pictures taken by AFP photographer Paul Ratje show Haitians going back and forth across the border to get food for their families, and finding themselves blocked by the horses.
“Some of the migrants started running to try to get around the horsemen, and one of the agents grabbed the Haitian in the picture by the shirt and he ended up swinging him around while the horse trotted in a circle,” he said of a couple of particular photographs.
US Border Patrol chief Raul Ortiz said the incident was being investigated to make sure there was not an “unacceptable” response by his officers, adding they operated in a difficult environment, trying to distinguish migrants from smugglers.
US Border Patrol agents on horseback have been trying to close off crossing points along the Rio Grande river
The UN’s refugee agency, the UNHCR, has said it is monitoring US expulsions to Haiti, adding it was imperative that people fearing persecution had access to asylum, Reuters news agency reports.
The US government has reported a surge of migrants arriving at the border with Mexico this year.
The number of people detained there in July exceeded 200,000 for the first time in 21 years, according to government data.