We have three OTTB’s and they will stand at the block but once your leg swings over they move off. It’s what they know from the track.
My greenie was like this. She hated mounting and would swivel herself away, ears pinned, picking up legs. Key point: There was some muscle pain - I had that released and found a saddle that fit her with her wide base of withers. Then we started training again.
What worked for me: first was a few weeks of me just standing on the block, petting, grooming, leaning, swishing a rope or a blanket. Lots of ground work where I learned she is vigilant about things on her right side… So when you swing your leg over, it goosed her a bit. So we worked on giving me the right eye
Next I trained her to move her feet with whip touches. If she shifted her hind quarters or moved away from the block, I tapped her back over. No matter what, she had to move her feet back to the block - and then a release of pressure. So the block became the safe spot. Standing still became safe spot.
The next phase consisted of a lot of sitting and doing nothing after I mounted. Sitting for 10-20 min. If she moved, I halted her. And we sat some more. We sat and sat and sat and did nothing. And then I got off. We still do this every time I ride - get up on the block, mount, and sit.
This all took about 6 weeks. She doesn’t move anymore.
To the people who suggest backing the horse up when he won’t stand at the mounting block- that can become a serious evasion. Some horses are MORE than happy to back all around the arena!
A better way is to ask for small circles- not quick or angry ones, just small circles with an offer to stand and rest at the block. N0? OK, small circles again in the other direction for a few minutes and again offer a rest at the block. Just be careful to keep everything calm but consistent. When you get a quiet halt at the block with a bit of a sigh, praise LOTS and end your session. Repeat and rinse daily until horse enjoys a peaceful time at the block before you try to get on, then carefully increase your request as others have mentioned above.
It is really worth a few days training to have a horse stand quietly to mount, don’t rush it.