Is the option to breed your horse available? It may be that there are no public coverings available (which is likely, given the covering limit), or you don't have any male donkey unicorns of the same breed.

It's important to note that there are two different breeds of donkey - Standard and Poitou - and unicorns can only be purebred so double check your mare's breed and see if you have any males that match.

Hope this helps!

Just double checked with one of my uni mares to answer the second part of your question: I couldn't see any coverings that weren't by unicorn males, so I believe that it has to be with another unicorn!

Sent you a covering for your Donkeycorn. There is only a 1/5 chance of getting one though so if you have a Hestias and a fertility wand in your BMI I would advise using them so you are guaranteed uni foals.

All the best.

Just to add for clarification, Donkey and Draught unicorns have a 2 in 5 chance to get a unicorn foal, compared to normal unicorn horses which have 1 in 5

Good luck with breeding

Got my answer. Not enough donkey unicorns