Ask Mr. Smarty Plants

By horse-resistant, do you mean that you are looking for perennials that are not poisonous to horses or do you mean that the plants are not eaten by horses or can withstand horse hooves on them? Or all three criteria?

The first place to go to find a list of potential plants is our Native Plants Database.  Use the Combination Search feature instead of Recommended Species. This will provide a bigger selection with much more choice to narrow down. The volunteers and staff at the Wildflower Center who maintain the database have partners in different regions to help with these recommended species lists based on what is easy to access in local nurseries.
Under Combination Search, select the following categories: State – Colorado, Habit – Herb (for herbaceous), Duration – Perennial, and Light Requirement – Sun.

These search criteria will give you an extensive list of plants (267 to be exact) to start. Follow each plant link to our webpage for that plant to learn its growing conditions, bloom time, etc. At the bottom of each plant webpage, under Additional Resources, there is a link to the USDA webpage for that plant. Take a look there for more specific details about suitability before you put them on your final planting list.
Next compare this list with the list of plants poisonous to equines. There are several on the internet.
List of Plant Poisonous to Equines on Wikipedia (this is a list of all types of plant, not just perennials)

Plants Poisonous to Livestock on the Cornell University College of Agriculture and Life Sciences website. You can search by botanical name and common name.

Toxic and Non-Toxic Plant List – Horse by the ASPCA. This list provides non-toxic plants which may be an easier search than excluding toxic plants.

For help with pasture management, contact the Colorado State University Extension. They have a small acreage management page on their website and a Colorado Forage Guide that you can download. It is Bulletin #563A.  Jennifer Cook is the Small Acreage Management Coordinator for Front Range at NRCS/CSU Extension, 57 West Bromley Lane, Brighton, CO 80601 303-659-7004 ext.3 Her email is [email protected].

Lastly, take a look at the hardiness of the perennial plants on your final list. Are they able to withstand your zone 2-4 winters?