"It has been explained to me that the Wild Maa was a pre-christian fertility symbol which became adopted as the symbol of one of KlienBasle's trade guilds in the 13th century, along with Vogel Gryph, and Leu."

All sounds sort of plausible but who explained it and what was the evidence?

Pardon my cheek but as Malcolm pointed out above:

"You are probably confusing the holiday 'Jack in the Green' tradition (see Georgina's post) with the unrelated 'foliate head' carvings that have appeared in churches since the Norman period. Nobody called them 'Green Men' until Lady Raglan in 1939 (Folk-Lore 50, 45-47), but ever since then the romantics have been imagining all sorts of extravagant things involving pagan fertility rites and the like."