Cholla - pronounced Choyah after a desert cactus - uses his mouth to hold a brush and paints on an easel-mounted canvas in his paddock.
Owner Renee Chambers, 51, of Reno, Nevada, said she first noticed Cholla's hidden talent four years ago when she painted the fence of his paddock.
She said: "He seemed really interested in what I was doing then one day my husband Robert joked to me that I should give Cholla a brush.
"He hasn't looked back since then, I give him the canvas and the paint and he just makes these wonderful pictures."
She said she squeezes the paint out of the tube for him, puts the paint on the brush and hands it to him, but insists that is the extent of her involvement.
"I have thought he might be the reincarnation of a famous artist, maybe Monet or someone, but the one thing he is definitely is a horse who loves to paint."
Rosalba Giorcelli, from Galleria d'arte Giudecca in Venice, which is to show an exhibition of his work in the spring, said she did not initially realise Cholla's work was painted by a horse.
She said: "We exhibited it at a recent art fair, and the reactions by the public and the artists were enthusiastic.
"They all understood we did not want to diminish any human artist's artwork, but we wish to contribute to a debate about contemporary art." (AP)