Hmm...
Look at screenshots:
http://img692.imageshack.us/g/mb322.jpg/Description of screenshots:
All screenshots were done with the same build and the same bow (nomad)
@1 Sumpter horse standing still
@2 Sumpter horse at full speed
@3 Courser standing still
@4 Courser at full speed
@5 Build *protip* look at ha skill *protip*
Why does it happen? Well if you don't know with HC and 5HA I hadn't such a problem. Hm...