Ask Mr. Smarty Plants

Since this is the Wildflower Center, Mr. Smarty Plants will address the plant part of the question first.

Agarita Mahonia trifoliolata (agarita) is a good choice for your barrier hedge. It can reach up to 8 ft. tall in favorable conditions, and its rigid branches can form thickets. This along with its spine tipped leaves should give you the bramble hedge you desire. An added bonus is the numerous red berries that attract birds. Because of its spiny leaves, Agarita is not a preferred broswing plant for livestock, and it is generally avoided if more preferred food is available.

Another choice that can form dense growth is Wax Myrtle Morella cerifera (wax myrtle). It is not prickly like Agarita, but is a fast growing, widely used screening plant with fragrant leaves and blue berries.

A third choice that is not as prickly as Agarita, but can be  denser than Wax Myrtle is Eastern Redcedar Juniperus virginiana (eastern redcedar). This evergreen usually reaches a height of 30 - 40 ft. but can be pruned to form a hedge. There are dwarf varieties available that would make this easier. Contact UConn Plant Database or North Carolina State University.

As to the question of toxicity, I've included links to five database on toxic plants.

 Texas Toxic Plant Database

 ASPCA

Equisearch.com 

Cornell University's Plants Poisonous to Livestock and other Animals 

Poisonous Plants of the Southern United States

Please note that the absence of the names of these plants from these lists does not guarantee that it is non-toxic to horses, but not finding it on the lists makes it less likely to be toxic. To search the lists, I recommend using the scientific name since those names are generally standard, whereas the common names often vary in spelling and usage.

Mahonia trifoliolata

Mahonia trifoliolata

Morella cerifera

Juniperus virginiana