Ask Mr. Smarty Plants

It would take a lot of space and a lot of time for me to list every plant with possible toxicity for horses in Travis County, but I can list some that are especially bad for horses or that are more abundant.  

First of all, here is an article from Equisearch with a list of the 10 Most Poisonous Plants for Horses.  Not all of those plants are common in Travis County or, when found, are not abundant. Here are the ones from that list that have been found there:

Conium maculatum (Poison hemlock), a non-native introduction.  Here is more information from Plants of Texas Rangelands.

Sorghum halapense (Johnson grass), a non-native introduction.  Here is more information from Plants of Texas Rangelands.

Astragalus crassicarpus (Groundplum milkvetch), Astragalus distortus (Ozark milkvetch), Astragalus lotiflorus (Lotus milkvetch), Astragalus nuttallianus (Nuttall's milkvetch), Astragalus plattensis (Platte river milkvetch), Astragalus pleianthus (Edwards plateau milkvetch), Astragalus wrightii (Wright's milkvetch) and Oxytropis lambertii (Purple locoweed).  All native.  Here is more information about Oxytropis lambertii (Purple locoweed).  Some of these are rare or don't occur in great abundance.

Nerium oleander (Oleander), a non-native introduction.  Here is more information from Plants of Texas Rangelands.

Centaurea melitensis (Maltese star-thistle), a non-native introduction.  Here is more information from Plants of Texas Rangelands.

Cicuta maculata (Spotted water hemlock), native.  Here is more information from Plants of Texas Rangelands.

The ASPCA has a list, Toxic and Non-Toxic Plants—Horses, that you should look through.  Not all those plants occur in Travis County or even in Texas.  You may see some names that you recognize, however.  [Note that the first part of the list is for "Plants Toxic to Horses" and the last part of the list is for "Plants Non-Toxic to Horses".]  Here are some that are on the list that do occur in Travis County:

Juglans nigra (Black walnut), native.  Here is more information from Pennsylvania Poisonous Plants Database.

Aesculus pavia (Scarlet buckeye), native.  Here is more information from Plants of Texas Rangelands.

Hedera helix (English ivy), a non-native introduction.  Here is more information from Poisonous Plants of North Carolina.

Nandina domestica (Heavenly bamboo), a non-native introduction.  Here is more information from Poisonous Plants of North Carolina.

Melia azedarach (Chinaberry), a non-native introduction.  Here is more information from Plants of Texas Rangelands.

Prunus spp. (Includes plums cherries, chokecherries, peaches, apricots, etc.), some native and some introduced non-natives.  Here is more information from Plants of Texas Rangelands.  There are domestic Prunus spp. (plums, peaches, etc.) but there are also native species that occur in Travis County:

Prunus caroliniana (Cherry laurel)

Prunus mexicana (Mexican plum)

Prunus minutiflora (Dwarf plum)

Prunus rivularis (Creek plum)

Prunus serotina (Black cherry)

This is an extensive list; but it isn't all plants that have some toxicity that your horse might encounter.  You should look through the list from the ASPCA and in Toxic Plants of Texas from Texas A & M AgriLife Extension for plants that might occur where you are pasturing your horses.  You can determine if they have been reported as occurring in Travis County by entering their scientific name (be sure it is spelled correctly) in the USDA Plants Database "Name Search" slot.  When you reach the species page, click on Texas on the distribution map to see the county distribution for the species.