Saddlebags

For whatever reason, I cannot give a donkey or a mule a saddle bag. I have tried this in survival & creative. I have made attempts by crouching & right clicking, and sitting on it's back and clicking, all while the animal has a saddle on. I can equip it with the saddles & name tags, but no inventory slots... I have also tried this on multiple computers with the same effect. if need be I'll include files for diagnostics (just tell me which ones).