Its discovery was made in the Poison Strip Sandstone Member of the Cedar Mountain Formation in Grand County, Utah, United States.

The generic name, Planicoxa, which means "flat hip", refers to the flat appearance of the ilium formed by horizontal folding over of the postacetabular process (rear portion of the ilium), the defining characteristic of the genus; venenica, the specific name, is Latin for "poison" in reference to the Poison Strip Sandstone Member of the Cedar Mountain Formation where the discovery was made.

The femora are standard for ornithopods, but the ilium has a short, horizontal postacetabular process that is functionally an antitrochanter[clarification needed].

The discovery of P. venenica added significant information to the Barremian-Albian fauna of the Cedar Mountain Formation.

A second species, P. depressa, was created by Carpenter and Wilson (2008) for material previously named Camptosaurus depressus by Charles Gilmore in 1909.