Mounting figures; you have two choices really file the inside of the legs on the rider down & perhaps the horse this is later is a better idea if the rider had the saddle moulded on & care needs to be taken not to damage any saddle cloths or other detail moulded on to the horse.
Or to fix the rider on with a small amount of fresh
green stuff as fresh green stuff is quite a good adhesive thought you do have to be careful not to use too much or if you do & is squeezes out form under the rider sculpt it away before putting the figure to one side to cure. I say
green stuff rather than
milliput as whilst they both work
green stuff is stickier & leads less re-gluing.
I would point out that mounted figures seldom fit on their horses without some work unless the figure & horse where sculpted as a set & the horse is not intended for other riders of the same kind which they often are.
Ginger