Texas urban folklore. This is a creepy one. Legend has it that there is a paranormal entity that lurks near the bridge on this road. The road is no longer used by cars. There are plenty of versions of the story behind the Donkey Lady, but most include a fire where a woman was badly burned and disfigured. This is probably where the term Donkey Lady came from. The legend says that she hangs out near this bridge, seeking vengence on the horrible tradgedy she endured as a young woman. There are many references to this urban legend online. A quick search will give you tons of information. If you are a ghost hunter, this place may definitely interest you.
Update 9/20/2020: I recently came across this article: https://sanantonioreport.org/the-donkey-lady-a-san-antonio-legend-now-haunts-the-phone-lines/ that describe the Donkey Lady Hot Line. If dialed, you may just get to talk to the Donkey Lady herself! *just another fun layer to this creepy mystery!
Additionally, there are miles of paved trails in the area, connecting this greenbelt to the ever-expanding San Antonio trail system. There is a sign at the trailhead that indicates the trail is closed after sunset. The cache is close enough to the trailhead/parking that you can see it and your vehicle from the cache location. Enjoy!