Because your standard orc just isn't awful enough.
Level 4 Archer (humanoid)
AC: 21 (19 dismounted) PD: 18 (16 dismounted) MD: 14
R: Composite Bow – +9 vs. AC, 13 damage
Even: Shrieking Missile– the next ally to attack this target gets a +2 to hit
16+: Felling Shot – target must use two move actions to move elsewhere nearby (save ends)
M: Sabre – +8 vs AC, 11 damage and the target takes a cumulative -1 penalty to AC until the end of the encounter
Steppe Cavalry: The Orcish Horse-Archer can move twice as fast as a foot soldier, getting to Far distance in a single move. In addition, the Archer can make its attack at any point during a move, freely disengaging and engaging opponents once per round as needed.
If a player character hits an Archer with a natural 18 or better, that character can choose to forego the normal effects of its attack to dismount the Archer. The Archer takes 15 damage and loses all benefits of the Steppe Cavalry feature. In addition, its AC and PD drops by 2.