Exotic Mount (Ex):
Small-sized beast riders can choose a pony or wolf mount at 1st level. At 4th level, a Small beast rider can also choose an allosaurus, ankylosaurus, arsinoitherium, aurochs, bison, boar, brachiosaurus, elephant, glyptodon, hippopotamus, mastodon, megaloceros, riding dog, snapping turtle (giant), triceratops, or tyrannosaurus. At 7th level, he can also choose a dinosaur (deinonychus or velociraptor).
at 4th level, our small cavalier upgrades from his CR1 Wolf that's been getting a little "long in the tooth" to a CR 9 T-Rex? If you're playing outside, that's a pretty decent mount. If you're inside, you don't get much out of a horse, either. Of course, why not bring the whole party along and get a CR 10 brachiosaurus.
I'll probably stick with T-Rex even at 7th, rather than switch to CR3 deinonychus, though.
Also, medium characters w/ this can ride tigers/lions (and it would be an unfair nerf to the RAW to deny mounted archery in such a case w/o a priori discussion)...but small can't...yet those big cats are bigger than riding dogs and significantly smaller than arsinoitherium...
