Anything in the MM that says Cohort in the Level Adjustment can be trained as regular mount. To buy a mount find the NPC level (14th or lower based on lowest party Cohort level) and look up the NPC Character Wealth and divide by 4 for the cost to train that mount.
|NPC Character Wealth by Level|
|1st||900 gp||11th||21000 gp|
|2nd||2000 gp||12th||27000 gp|
|3rd||2500 gp||13th||35000 gp|
|4th||3300 gp||14th||45000 gp|
|5th||4300 gp||15th||59000 gp|
|6th||5600 gp||16th||77000 gp|
|7th||7200 gp||17th||100000 gp|
|8th||9400 gp||18th||130000 gp|
|9th||12000 gp||19th||170000 gp|
|10th||16000 gp||20th||220000 gp|
For a Paladin special mount, you are limited to Heavy Warhorse if you are medium.
We will have a special house rule for alternate mounts of equal ECL to that of the special mount. This will take away powers if you use something with higher than 4HD or a LA.
The Giant Owl is a 4HD creature with a +2 LA (cohort). Make him your mount at a loss of 2 bonus HD and use the chart from paladin mount for 11-14 for paladin mounts.