You con not tp a horse to a location when you sit on it:
sit on a horse and type:
tp @e[type=horse,r=10] ~ ~10 ~
It will not work
tp @e[type=horse,r=10] @e[type=pig,c=1]
will work
ps: levitation willl also not work when you sit on a horse... the same problem i found by boats and minecarts
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However, this issue is a Duplicate of MC-97513.
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