You imbue a tiny creature with the power of nature, and it grows to such a size that you can ride it.
Fey Magic Gift 840 gp
Utility Power (Polymorph) Daily (Standard Action)
Effect: Choose a nonhostile Tiny natural beast within 5 squares of you that is on a surface that can support a Large creature. Until your next extended rest, the target becomes Large, gains the mount keyword, and acts as a willing mount for you. The creature has the same statistics as a horse (Monster Vault).
Published in Heroes of the Feywild, page(s) 142.