*shrug* I was assuming that Leadership let you pick a nonstandard creature that could then double as your paladin mount.
Still, a sylph is a faerie creature. I don't picture her bending over and letting a paladin straddle her. (All-female race?)
It cast spells like a 7th level sorcerer, so it uses alterself to take the form of a hell hound.
I was irritated by the fact that you ignored my question about the sylph's type and alignment, so I looked it up. The sylph is an outsider native to the elemental plane of air with a usually neutral alignment. A standard sylph has a 15 Intelligence.
So, while it is
possible that your paladin could contact a sylph from the elemental plane of air, and it's
possible that you could find a sylph that's either Lawful Good, or otherwise willing to join your cause (and hurt your Leadership score for being of a different alignment), and it's
possible that you could convince her (all-female race?) to transform into a form that's suitable for riding,
none of that is very probable.
It would make for an interesting, if slightly erotic adventure, however.