BibliOdyssey, which you can now follow on Twitter, points to a wonderful collection on Pinterest by user "Iki" of images of "Imaginary gardens in art: from unicorns to mandrakes & from Botticelli to Blake." This particular image featured above, from that Pinterest set, comes from "Medieval Mondays: Wild men in Medieval folklore," a post on the blog of A.J. Walker.
"Medieval Europeans were fascinated by what anthropologists call 'liminal zones,'" writes Walker, "Areas crossing from one state of existence to another, in this case from civilization to wilderness."
There was a lot of wilderness in medieval Europe, and since most people didn't travel, this wilderness was looked upon with wonder and fear. Who knew what might be living in that primeval forest? At the edge of human habitation there certainly were some strange people: bandits, hermits, madmen, so perhaps there were monsters too.
Medieval society was a strict and hierarchical one. Everyone had their place and they better stick to it. In the more rural areas, though, the church and state had less of an iron grip, and people could get away with more. Time and again in the historical record there are reports of rural people engaging in rituals that look like pagan survivals or revivals. These were dangerous but exciting, and medieval people looked upon these remote regions with a guilty thrill. The wild man is a projection of this.
Images of wild men are so frequent that some have argued that they may have been real. Some say there may have been primitive tribes living in the more remote regions, or even surviving Neanderthals. There's no evidence for this. I think there probably were a few wild men, people who left society either by choice or by force, who lived a semi-wild life in the woods, wearing uncured pelts as clothing. They may have been a danger to farmers living on the edge of civilization, stealing livestock or women and children as is often depicted in wild men imagery. In the weird, wonderful world of the Middle Ages, it's not unlikely.
Also, you can read this book online: "The Wild Man: Medieval Myth and Symbolism," by Timothy Husband and Gloria Gilmore-House (1980), published by the Metropolitan Museum of Art, New York.
Check it out in the manuscript viewer.
The excellent website Public Domain Review has more on this image collection.