It has been prohibited to ride the beast which eats dung.  

عَنْ رُكُوبِ الْجَلاَّلَةِ  

The Messenger of Allah ﷺ prohibited to ride a camel which eats dung.  

نَهَى رَسُولُ اللَّهِ ﷺ عَنِ الْجَلاَّلَةِ فِي الإِبِلِ أَنْ يُرْكَبَ عَلَيْهَا