I'm sorry, there aren't any public My Little Pony mods in NA, at least. But there are ways you can create this mod. The first of the two I'm going to mention being harder than the second.
A) To take models from other games and add them to Mabi you'd need to first extract that model (or find a mod of some other game) then find a tool to convert that file type to a pmg. Then reanimate/create a new framework for them. However, in this case, you'd need to edit racedb.xml to account for a nonexistent custom race type/animation set and rename the framework/animations you made with that. However, that would be exceedingly difficult for the average user and I don't believe there are any guides on this website on how to do that.
B) I suggest this alternative. Take the miniunicorn PMG (the miniunicorn will be normal-sized if you use it to replace it over a normal horse), then you could edit the texture used or create a custom texture+edit an xml file in the _define subfolder to make it look like a My Little Pony character then remove the horn with a PMG editor if you wish. (You might also wish to use the PMG Color Editor to alter the color overlay over the texture.) For guides on ordinary PMG editing, check the stickies of the skin modifications or guides section.