Hello,
First off I would like to thank you for being considerate of your neighbors with horses as many trees and ornamental bushes are deadly to horses. So thank you for thinking of them.
I have gone through my trusted sources and books and this is what I have found.
In the Euonymus family the burning bush (e. atropurpureus) is toxic to horses, but I didn't find any others in that family that are toxic.
I did not find anything about the viburnum except sweet viburnum as being toxic.
Juniper is safe for horses, but not cattle. Many evergreens contain a substance that causes abortion in cattle if the needles are consumed. With that said, I don't know if any research has been conducted regarding abortion in horses if the needles/foliage are eaten. Unless horses are starved, foliage from evergreens aren't appealing to eat. I know many people in my home state of Montana that have juniper in their horse pastures with no problems.
I researched the abelia, snow bush and wax myrtles in 4 of my trusted sources and they didn't appear anywhere as being toxic.
Trees such as red maple are toxic to horses as when the leaves wilt and fall they become sweet, which is appealing to most horses as they often do have a sweet tooth. The leaves contain a toxic substance that is deadly.
I think a conifer/evergreen type tree/bush is safer to plant than a deciduous tree when it comes to horses and even cattle.
Please let me know if you have any other questions. If you have more questions or need more help please don't hesitate to email me directly at the email listed below.
Happy planting.
Dawn