What breed of horse was TV's Mr. Ed?

Palomino is a coat color in horses, consisting of a gold coat and white mane and tail. Genetically, the palomino color is created by a single allele of a dilution gene called the cream gene working on a "red" (chestnut) base coat. However, most color breed registries that record palomino horses were founded before equine coat color genetics were understood as well as they are today, and hence the standard definition of a palomino is based on the coat color visible to the eye, not the underlying presence of the dilution gene.

Due to their unusual color, palominos stand out in a show ring, and are much sought after as parade horses. They were particularly popular in movies and television during the 1940s and 1950s. One of the most famous palomino horses was Trigger, known as "the smartest horse in movies," the faithful mount of the Hollywood Cowboy star Roy Rogers. Another famous palomino was Mr. Ed (real name Bamboo Harvester) who starred on his own TV show in the 1960s.

https://en.wikipedia.org/wiki/Palomino

While Bamboo Harvester, the horse who played Mr Ed, is usually referred to as a Palomino, that is only a description of his coloring. According to the pedigree at the link, his father was a saddlebred (at least, three-quarters so) and his mother was part-Arab. He was 'just a horse' as far as breeding goes.

https://en.wikipedia.org/wiki/Bamboo_Harvester