Hero replacer.

You don't even need to understand scripts. Open the mod as an archive (using WinRAR, for example), find the horse image (in .dds format) there (just browse around, you'll find it quickly; there are characters and other stuff). Then, find any program online that allows you to open and edit .dds files (it's a type of image format). That's it. Open the program, edit the horse image . No scripts, just artistic talent

UPD: You can actually use a mod from this thread as a base, then replace the horse model in each character's folder with your own (Isabelle's), and then manually edit each one . This way, you'll have an up-to-date mod with different characters, Isabelle's horses, and different horse colors. You won't need to code anything, just edit pixels. It will take some time, but you won't have to think too much