Mild incompatibility with JEI and a very much needed tweak
So on the first part of the title, with JEI installed the horse/donkey GUI becomes very cumbersome to use. The little model preview for the animal along with the saddle/armor slots overlap with the JEI GUI, not only causing graphical issues but also making adding/removing saddles/armor very cumbersome. You can very easily shift+click the equipment onto the animal but when trying to take it off it'll usually try and fetch the recipe for whatever item is under the cursor on the horse GUI. The second part is less of an actual bug and just more of a major issue with the mod, that being the inability to remove ender chests from animals that have them. I understand why this is as you can't remove regular chests from donkeys in vanilla and I'm guessing that this mod uses more or less that system to function, the issue is more that the ender chest is a much more expensive item than a regular chest, and in the case of donkeys, it's not even a direct upgrade from the regular chest despite being permanent. I feel like this is especially problematic with horses and donkeys having random stats, making it possible to potentially cripple an extremely good donkey by misclicking with an ender chest.