Schoonover illustrated the first book, The Bar-20, that created the iconic cowboy, Hopalong Cassidy in 1905. The author, Clarence Mulford collaborated with Schoonover on naming, and subsequently creating an image of a gruff cowboy with a limp.
Schoonover suggested, Hopalong. Mulford added Cassidy and the rest is history. Schoonvoer illustrated three books in the Hopalong series.