Yesterday I went horseriding, went on a old pony who is as moody as anything! We were doing open order, she ended up cantering everywhere, with me nearly touching her back since I was trying to get her to stop! She was overtaking all the horses trying to bite them etc. Then we started jumping, refused them all and was jumping at the wrong time.

So my question is, is this the horse doing this or me? I'm 100% fine with other horses.

Neither.

The riding school should not use the old pony in lessons if she's 'moody as anything' and they shouldn't have asked you to jump her when you were already having problems. The old girl probably has pain issues and is probably sour.

I would suggest you find a different school to ride at.

I agree with Elles, it sounds like the pony is not happy, probably due to pain