When creating a new cavalier, the mount options should only include medium-accessible mount options when the character is a medium character, and small-accessible mount options when the character is small. The list currently includes all regardless of character size.
Additionally, if the cavalier is 4th level or higher, he is allowed to choose boar or dog as well. From Paizo's website on the cavalier page:
"The creature must be one that he is capable of riding and is suitable as a mount. A Medium cavalier can select a camel or a horse. A Small cavalier can select a pony or wolf, but can also select a boar or a dog if he is at least 4th level. The GM might approve other animals as suitable mounts."
OK, I was wrong about the 4th level part. They are being added to the selectable list. However, they should only be selectable by a small sized creature.
Completed: At revision: 17629 - Fixes Size Issue
Completed: At revision: 17631