Mounted archery is a very expensive class to maintain, many horse archers believe that you need a HA skill of 4 to be a viable horse archer, and this leaves very little skill points to put elsewhere. This is an example of a HA/polearm build
Strength: 15
Agility: 24
Hit points: 50
Skills to attributes: 8
Ironflesh: 0
Power Strike: 0
Shield: 0
Athletics: 0
Riding: 6
Horse Archery: 4
Power Draw: 5
Power Throw: 0
Weapon Master: 8
One Handed: 1
Two Handed: 1
Polearm: 111
Archery: 151
Crossbow: 1
Throwing: 1
You can play around with WM and Riding skill, maybe you would prefer 7 and 7 instead of 6 and 8 but as you can see there is very little flexibility in this build. A more flexible build would be:
Strength: 15
Agility: 21
Hit points: 50
Skills to attributes: 2
Ironflesh: 0
Power Strike: 0
Shield: 2
Athletics: 5
Riding: 7
Horse Archery: 3
Power Draw: 5
Power Throw: 0
Weapon Master: 7
One Handed: 1
Two Handed: 1
Polearm: 111
Archery: 151
Crossbow: 1
Throwing: 1
With this build you're a viable footman and you can use a shield with your lance if you so wish, however with only 3 HA skill you might want to put more WPF in archery as your accuracy will really suffer. Another good option for this build is to go mounted-xbow because although you still need HA skill to use xbows on horseback, it's not nearly as important as it is for archers.
If you are new to cRPG I would highly recommend you go regular archer first and make a 18/21 build or a 21/18 build.
*edit this WPFs are roughly chosen. I'm not recommending putting exactly 151 points in archery.
