Adorious is right, you would need new sprites. That's maybe the "easiest" part depending on your experience developing OT servers, then you would have to create a "system" so the players can "mount". For this you could try using the existing functions (It shouldn't be hard I think) or develop a new dll (which would take you more time) and then develop the lua scripts based on this dll.
An easy way to implement this maybe is creating items a player can use to "mount". For example if I use an spool of yarn I will mount a giant spider. But you would need to add new sprites no matter which of the methods you pick. If you don't have experience with programming you could maybe check some online books (there are tons of them with good content) or just pay somebody to do this for you. Good luck!