Horse Stuck in Box Stall

@ImaGamer96  Bring up the cheats console and enter "resetsim firstname lastname" without quotes, using the first and last names of the horse.  Try it a couple of times; occasionally the first reset doesn't work.  Once your horse is elsewhere, be sure to delete and rebuild the stall.

If that doesn't work, go to Edit Town and evict the horse, then place it back in the house.  I'm not sure whether you can move an animal out alone (I've never tried it), but you could split the family and move only one sim out.  The sim that's temporarily evicted will keep any job and all relationships but will lose promised wishes and opportunities.

Be sure to delete your cache files as well, each time you quit, at least until you've resolved this issue.  The files should be deleted regularly anyway, as they tend to clog up with stale data.  For reference, the files are:

• CASPartCache.package
• compositorCache.package
• scriptCache,package
• simCompositorCache.package
• socialCache.package