[oergiR]

In English everywhere except the US, it’s “horse riding”; “horseback riding” is US English.

Definitely humorous: https://m.youtube.com/watch?v=5wSw3IWRJa0

[OJFord]

I know it originates in the US (where I'm not from, perhaps hence it grating) but like so many things it does seem to be catching.