Some less common ones are 'Lipstick' ornamental strawberry and Taiwan raspberry. They do both produce tiny little fruits but they are not very good eating quality. What if you planted a few watermelon seeds?

If you are looking for a bush, Frankincense (Boswellia sacra) is extremely drought tolerant, kind of spiky so your horses will likely leave it alone, though not poisonous, and very good smelling when the plant is cut and leaks out sap.

Portulacaria afra 'prostrate form' would be a good groundcover and it's a valuable forage for horses.

I like the ceanothus suggestion. After it establishes it may not need summer watering. If you want something lower Vinca is tenacious. Don't know about toxicity.

Change "may" not need to "will" not need summer watering (after establishment) and you've got it for the ceanothus! Vinca would cook in that situation - I wouldn't bother with that.

• "performs best in rich, evenly moist, well-drained soils in partial shade, but is adaptable to soils of average fertility, soils of various pH, and occasional drought (once established), but is not tolerant of full sun (which quickly leads to decreased vigor and chlorotic foliage)"

Vinca thrives in a southern exposure here. A weed almost.

Southern and central coast CA have been in severe drought for 5 years and some shrubs which previously subsisted on winter rains have been seriously stressed.

Santa Barbara daisy (Erigeron karvinskianus) is another to consider. Reseeds easily.

E. karvinskianus can be extremely aggressive.

irisgal, that's pretty interesting. Up here in the less intense (and much lower heat) summer sun, vinca struggles mightily and is typically a washed out color rather than a deep, rich green. I would not recommend it for a dry, full sun location.

I have sedum album as a ground cover. It spreads well and can be started with just a minimal cutting. Looks great under and around succulents. Maxs out about 6" but is usually much lower. It flowers white and the bloom stalks can just be left or I often just pull them out. The Santa Barbara daisies aren't a bad idea either, but they can be muchas invasive. I had one plant and now I weed them out of everything.