Regarding this autumn/winter season, there are really several brands that I can suggest you. Big names like Dior proposed us an incredibly beautiful collection, that is also incredibly equestrian! One of my favorite is Tory Burch, an American designer and horse lover who dedicated a full collection to the equestrian world with original interpretations. A new and very interesting brand is Miahatami, born from the creative mind of a young Iranian fashion designer clearly inspired by Persian art.
Also the English brand, Hobbs London, much loved by Kate Middleton, created an amazing collection in collaboration with Historic Royal Palaces inspired by Queen Victoria love for horses. And, well, the English designer Stella McCartney is a horse lover too and proposes very often equestrian-inspired items in her collections.
Fashion is a world always evolving :-)