If you are mounted when you cast mirror image, do the images also have mirror copies of your horse, or would their be 5 images sitting in the air around you?
2\$\begingroup\$ I've removed the positioning question - that's unrelated to the mounted casting issue, and should be asked separately in its own question. You can still retrieve its content in your revision history, which you can access by clicking the 'edited X time ago' link at the bottom center of your post. \$\endgroup\$– doppelgreenerJan 1, 2016 at 4:50
Exactly what it says: it is a personal spell that creates images of you.
As such, the spell is trivially defeated by saying "I attack the image on the horse"
If the horse is your familiar (or similar) then the spell can also affect it.
1\$\begingroup\$ Well done spotting the Familar exception! \$\endgroup\$ Jan 1, 2016 at 12:38
2\$\begingroup\$ Does it make any difference that "For simplicity, assume that you share your mount’s space during combat" (PH 157)? \$\endgroup\$ Jan 1, 2016 at 13:47