How to deal with a uncastrated stubborn mini donkey that isn't mine

I'm currently renting land from my husbands boss to pasture my pigs.  To get access to the pasture I use thou I need to pass thru a dry lot pasture that houses a neighbors miniature donkey. They are not very attentive owners so this donkey is completely intact and not at all handled with his halter always on cause they cant catch him to put it on otherwise. He is fed and watered so its not abuse. Just I dont think they know anything about donkeys and thought he would be sweet and cuddly. He is NOT sweet and cuddly. He is a bored and angry ball of lonely and untrained donkey fluff stuffed with pent up hormones! I honestly wouldn't have the time to care if I didn't have to pass thru his space. Of course he runs up and waits for me at the gate when ever I come up. Then will act friendly until he isn't then he rears up and trys to bite me and wont leave my space. I have limited experience with well trained happy horses and none with untrained angry donkeys. Any ideas on how to maybe just train him enough to stop trying to fight me without causing any more behavioral problems for the donkey or me.  Is it something Im doing? Or is he just territorial with nothing to do so Im the coolest punching bag.

And yes I'm looking into another gate to avoid this altogether but its going to require a bridge or access road and culvert to do so and I'm not sure its worth it on property I don't own.

Donks, especially mini donks, are just horse puppies.

For your situation, I recommend bribes. Donkeys can be territorial over their space but that doesn't excuse them from having bad manners. Carrot or apple pieces may be accepted donkey currency to 'pay' your way through his space.  

I'm discovering most livestock are just weird shaped puppies lately! I bribe him with some handfuls of grass but maybe a apple chunk or two would be better. I am worried about him getting pushy trying to get more while I'm halfway thru the pen....

I agree,  treats should help calm the savage beast.  
After all, freshly-baked cookies will change my attitude any day, should work with a lonely donkey.

I'll give you another thing to look for in your future interactions. Watch his ears as he interacts with you. If they are pinning back, he is NOT happy. If he is hopping and kicking but his ears are staying relatively forward he may be trying to initiate play with you.

Donkey play can be hazardous as they think kicking and rearing is the bees knees. You may need to ignore (don't turn your back on him!) and carry on with confidence until he 'gets' it.

I'm hoping the bribe does the trick but you might unintentionally be becoming his buddy.

I have a feeling your spot on. I also worry I'm going to end up owning said donkey with the way the situation is.
I haven't had time to look at his ears to much when he rears as Im normally worried about his feet and bulk coming at me. Il check the ears. Il feel better if hes just trying to play and not angry with me.