I use a slightly different macro from Djad's version and I'll admit I've never tried the target horse mount option, so I don't know what its limitations are, but I use the following:
1. Target nearest follower
2. Use targeted object.
With this version of the macro, I can mount any rideable pet almost instantly as long as I am standing near enough to use it. If I have two rideable pets out of the stable, I just make sure I'm standing closer to the one I want to ride.
I find this macro very useful for my tamer because the macro works with my Cu's, horse, nightmare, etc...
Of course you would still need a different macro for an ethy, as Merlin stated above.
Just throwing a different option out there. DjAd's version will also work just fine.