Is a woman allowed to ride a horse?
The Gemara in Pessachim 3a and Rashi there indicate that a woman sitting on a horse puts her into a position which displays immodesty. Nevertheless there is no codified prohibition against a woman riding an animal and various sources in Tanach and Chazal indicate woman did in fact ride animals. In a group of woman and girls there is certainly no problem. When there are men present sensitivity should be shown to tznius concerns. If a long skirt is worn and covers the length of the legs this would seem to alleviate the main concern. When there is a need to ride horses, as for various forms of therapy, there is even more room for leniency.