You did a commendable job of describing "flower-of-an-hour," or Hibiscus trionum. Other common names are Venice mallow, bladder ketmia, modesty, shoo-fly, and rosemallow.
It is grown as an annual in gardens for its lovely but fleeting flowers. Unfortunately, it has become a weedy pest in cultivated fields, orchards, and rangelands. H. trionum is a member of the mallow or Malvaceae family along with common mallow (Malva neglecta) and velvetleaf (Abutilon theophrasti). These plants self-seed readily, and the seeds can remain dormant in the soil for years. In some states, H. trionum is classified as a noxious weed.
There were no immediate references to this plant being toxic to horses when we checked these university (Cornell, Indiana, and Illinois) Web sites listing poisonous plants. The Cornell Web site provides links to all of those resources (www.ansci.cornell.edu/plants/index.html).
