Yes these could very well be for horses, especially if they are all between 2 foot and 3 foot high (feet and inches are the usual terms for all horse jumps). They could be cross country obstacles for eventing. Eventing is the term for either dressage, show jumping, or cross country, or more usually all three in combination (also known as horse trials). However these can be set up for sponsored rides as well - which is cross country where you raise money for charity.
These can be anything from telegraph poles, to trimmed hedges, to tree trunks, to small poles held up, to actual gates / fences. I've done these before and even jumped an RAF glider. These can also include drops (vertical leaps down onto grass or water) or ditches (jump the ditch) or also water to traverse.
Sometimes these are just convenient natural items though or have been placed for other sports and simply utilised by riders, and have not been placed deliberately by anyone for the sake of eventing but have been utilised for convenience. There is every possibility these are for another sport entirely, or to prevent trespassing.