What is the complete phrase for birds of a feather?
"Birds of a feather flock together."
The phrase about horses is actually, "That is a horse of a different color," not feather. The feather animal phrase is, "That is a bird of a different feather." The horse of a different color was in the movie, "The Wizard of Oz:" and the phrase means some kind of unrelated or incidentally related matter with a distinctly different significance.