Horse riding and other similar activities such as riding a bicycle are problematic when hijab is not properly observed or for example it attracts the attention of na mahram or it is something out of the ordinary in that area. For example in Iran, it is not normal to see women riding a bicycle in the open streets however in the UK, it's absolutely normal and the clothing worn by the women would also not attract any attention therefore in the UK, riding a bicycle it horse riding in the open would not be problematic.

Horse riding and similar activities are subject to conditions which either make that permissible or impermissible to practice