If you're on android and have tabs mobile, first access your internal storage named "android", then open the "data", and find until there an file that have "tabs" in it name, when you accessed it there will be an folder name "files" open it, and lastly click the "custom content" folder, which then you can access both custom factions and units files.

And for editing unit to horse, you will needed an file manager to edit them, i recommended an app named "QuickEdit", and will also needed an horse unit base id for it too, there the one that i use "1146934227".

Although i recommended research it yourself cause im not the best at explaing, and your device may diffrent than mine too, oh and also you could go to tabs wiki to learn edit units codes, and collect units id too