A seat on a bicycle, motorcycle, etc. (Cambridge Dictionary)
A leather seat on a horse. (Cambridge Dictionary)
A seat fastened on the back of a horse or other animal for riding, typically made of leather and raised at the front and rear. (Oxford Dictionaries)
A girthed usually padded and leather-covered seat for the rider of an animal (such as a horse). (Merriam – Webster)
Saddle meaning on Longman.
Bicycle saddle. Wikipedia.