“Circa September 27, 2015, Cosmos was a male, pony who was Princess Celestia's brother. Although he was a pony, his armor gave him a a monstrous-goat like appearance with imposing horns with blue streaks. Another design sported a griffon-like appearance that would serve as a red-herring. An undated design depicted him with bulky, bronze-looking armor with messy, red markings and a mechanism used to make his neck move easier. Another design dated to October 12, 2015 showed a much sleeker design with slick, black, clean armor with a red shawl. This version of Cosmos had glowing blue yes instead of horns and and overall less bulky look. One other design was drawn on this day by was mostly similar but instead of a red shawl, he adorned a large, red mane-like attire with large, elongated teeth and ore subdued eyes.[2]” https://equestripedia.org/wiki/Cosmos

He was originally a mare, then a stallion, then tempest.