Ask Mr. Smarty Plants

Unfortunately, there aren't any roses native to your part of Texas, but I can offer a few prickly shrubs, trees and cactus bushes that are native to Fort Bend County or adjacent counties.

SHRUBS:

Erythrina herbacea (Coralbean) grows to 6 feet but usually dies back in winter (depending on the winter low temperature).  It's long branches are very thorny and would certainly discourage anyone trying to get past them.  It has beautiful red flowers in the spring.  Here are more photos and information from Aggie Horticulture, the School of Biological Sciences at the University of Texas and from the Florida Native Plant Society.

Ilex opaca (American holly) is evergreen and has very prickly leaves.  It is a slow grower but can reach heights of over 30 feet.  There are, however, dwarf cultivars (e.g., Ilex opaca 'Maryland Dwarf') that grow to only 3 or 4 feet high. 

Acacia schaffneri var. bravoensis (Huisachillo) grows 4 to 12 feet tall with thorny branches.  Here are more photos and information from Texas A&M Agrilife Research and Extension Center at Uvalde.

 SMALL TREES:

Aralia spinosa (Devil's walkingstick) can grow to 12 or 15 feet all in one season and dies back in the winter.   It has very prickly stems and leaves.  It really is a small tree rather than a shrub.  Here are more photos and information from Duke University.

Acacia farnesiana (Huisache) is really a small tree growing to over 15 feet.   It is semi-evergreen and has straight spines on stems.  Here is more information from University of Arizona Pima County Cooperative Extension.

Crataegus texana (Texas hawthorn) can grow to a height of 20 feet but is armed with 1 to 3-inch thorns.   Here is more information from Aggie Horticulture.

CACTUS:

You might also consider cactus such as Cylindropuntia leptocaulis (Christmas cactus) or Opuntia engelmannii var. lindheimeri (Texas pricklypear).  Both can grow as high as 5 feet and are certainly armed with impressive spines.