My question is similar to Liz. While I resolved the test question "Ce matin, ________ monté au grenier pour ranger un peu." by acknowledging that you dont 'climb the attic' but rather 'climb?? into the attic' and therefore needs 'ETRE', I cannot convince myself re the sentence "I got up on my horse".
If you translated as he 'I mounted my horse" then J'ai monté mon cheval.
But visually and maybe literally "i got up on my horse" is the difference between the dashing hero Lone Ranger style who really mounts and and the bad-guy Jack Palance who slowly 'gets up on his horse' and therefore needs time to "il est monté".
Ok I am being silly. But would you translate the english sentence "i got up on my horse " exactly as you would "I mounted my horse" ? Sad if true because then in french you would lose something in the transaltion.