How do you keep a miniature donkey from eating all the plants & trees?

A fenced in area for the donkey. Donkey's that are loose are going to do what donkeys do.

Put him in a corral away from the plants and trees.

Give him something to do. They are intelligent and need stimulation. This is most important!!

Do not feed him rich food. They are desert animals and will get too fat easily which is not good for a miniature.

I love donks!!

Donkeys eat meadow grasses and "weeds", but they also will eat tree leaves and even tree bark. They also love things like carrots and apples and, as you have learned, your ornamental lawn plants. There may be plants that donkeys won't eat but I don't know what they are and haven't been able to find a list for you. When the donkeys come back into your yard, you could watch to see what plants they avoid and then fill your yard with those plants; but I think what you are really going to need to do is to find a way to exclude the donkeys from your yard—or, at least, from the plants you don't want them to eat.

Does he have other equine companions? They are herd animals. Put him in a pen when you are gone and out when you are home to watch him. If he is bored he will eat anything. Give him horse treats, not dog treats. No sweet feed or grain. Give him a horse ball - a ball with a handle on it - teach him to play with it. Learn to ground drive him - teach him to pull a small cart. They are smart, curious animals, and do well with a job. They are not lawn ornaments.

They leave a number of our plants on our 5 acre lot in the Caribbean alone. sometimes they will it eat up an abundant plant that they have never bothered with ..if we transplant one! Go figure! I am going to try some Tabasco sauce tomorrow on a new transplant. They love our vegetable and fruit recycling program of leftovers.They are very smart , respond to yelling if caught , hate the hose and our Scottish terrier security guard keeps them away from the plants near the villa and the front entrance to the great room. We love them and the feeling is mutual🤪

Fence the Donkey in so he can't get to the the plants!

I would either corral the donkey or stake him to an area where he can do no damage.