Band of Bastards
You may acquire other critters that are improved as well, not just mounts.
A mule costs 25 gp, not 8 gp. Its movement speed is the same as a light horse, but they are ornery and can be difficult to train, resulting in a -2 penalty to Handle Animal checks and a -1 penalty to Ride. Mules also receive a +2 bonus to forced-march checks.
You may purchase a better mount by paying extra. For every multiple of the base price, you can add +1 Hit Die. This may increase the mount’s ability scores, etc.
If you can find them, you may have them. These mounts include hippogriffs, gryphons, pegasi, and a myriad of others.
This template is found in Eberron Campaign Setting, which adds a magical touch to a normal mount, beast, or critter.
The Warbeast template is found in Monster Manual II, which basically gives your mount more elite training. It’s broken, but it’s cool, so have at it. I may allow you to apply it to an existing mount between adventures, signifying time and money spent training your animal.
Want a fiendish pegasus? Go find it. Pay ridiculous amounts of gold. Want a fiendish warbeast magebred pegasus? Pay more.