In my current Pathfinder campaign, I have been playing as a Gnome Cavalier that has earned enough experience to level up for the first time AND has successfully reared a wild Vulture. One of the feats that has caught my eye is the Undersized Mount feat. Through technicality, my character (being small) could ride on the back of an other small creature (like my Vulture) if this feat was taken. However, I wish to be as close to RAW as possible, and have not been able to find any mounts smaller than Medium. As such, I feel that I must ask this; Is there such a thing as a Small Mount, or would it be useless for a Small Character to take Undersized Mount?
Small mounts exist in official material
PFSRD entry on Ride lists a great number of creatures suitable to be mounts (though the list isn't exhaustive) and some of them are Small. While indeed the majority of them are Medium or larger, you can find Small mounts that would perfectly justify a Small character taking Undersized Mount feat.
In fact, right there at the end of the list, in Other Animals section, you can find that Vulture is an animal suited to be ridden and indeed small. Go ahead and show it to your GM, the list specifies increased costs and that some rare vendors might even provide a combat trained Vulture!