Hmmm, as far as I know you'd have to get your mount the Spring Attack feat-- or Fly-By Attack, if it were a winged mount... so as far as Spring Attack goes, the mount would have to of course have Dodge and Mobility first, and a Dex of 13+. If we're talking about a light or heavy warhorse, this build is not going to work normally-- both the light and heavy warhorse have the Dex requirement but only get 2 feats. You would have to find an advanced one in order to get the third feat slot. A trained tiger, on the other hand... (just a thought). A pegasus already comes with Flyby Attack, and a griffon or hippogriff can have this feat if you switch out one of their usual feats.