In my opinion Italy offers one of the best fashion and equestrian brands in the world and a lots of companies are doing a great job such as Kep Italia, Cavalleria Toscana, Vestrum and the very new Ublack. However the Italian horse culture still has a very big gap and need to be developed as equestrianism is limited to a very few people and the path to become a professional rider is very hard. Too much compared to the German neighbors.
I also love very much the English style, at the stable I wear a Barbour jacket since my childhood! And the Palace Collection created by Hobbs London in collaboration with Historic Royal Palaces inspired by Queen Victoria love for horses is simply amazing, very well detailed in fabrics and prints. The English designer Stella McCartney is a horse lover too and proposes very often equestrian-inspired items in her collections.
US also offers a lot in fashion, California is probably the most active country and you can find a lot on Instagram… bloggers, riders, brands. I have lots of American followers.