I've found a post that may help you with this problem:
Compiling - Custom Client + 8.60 Mounts?
In this thread, HalfAway talk about Object Builder. This is the tool I'd recommend you to edit and compile your own spr/dat files. Unfortunately, the GitHub page of ObjectBuilder and other awesome tools has disappear or something, so I can't provide you a download link. However, you may have a look on the tools related threads on these forums, and maybe you find it somewhere.
As this user said (below), maybe it will be easier to make a 8.70 server, where you'll have the mount system already implemented. You will avoid tons of troubles using this, trust me.
On the other hand, if you are still interested on the 8.60 client version, look for the ObjectBuilder download and read these threads to learn how to use this tool properly:
Configuration - Adding New Outfits / Looktypes [Detailed + Free Samples]
[video] Importing custom sprites
[How To] Import your own Sprite to Server and Tibia Client [Video]
I hope this helps you.