Thanks everyone for the advice. Sorry its taken so long to respond but things have been hectic here lately.
So my pasture presents expensive problems to completely separating these guys, as the stables (the only shelter) are directly in the middle of the pasture. This makes things kind of difficult to divide.
I have already set up multiple hay stations that aren't visible to each other. That way Scarlet (horse) won't be able to see/protect the other hay station while she is eating, should she be so inclined.
I like the idea of making zones where the donkey can enter but Scarlet can't, so we are in the process of modifying one stall so that the bottom can remain open while the top remains closed, thus keeping out the big horse. I also want to look into placing some kind of similar barrier on one corner of the pasture so that the donkey can retreat to an exclusive grazing site if she needs to.
I'll keep an eye on it, and if the problem continues I suppose we will have to bite the bullet and do a more substantial reorganization of our setup, but hopefully this plan will work!
Thanks again everyone!