Wood Specter is a Beast monster type introduced in patch 2.4, only encountered on the Greyhollow Island in Act V of Diablo III Adventure Mode, not to be confused with Wood Wraiths.
"Is it right that I bent the will of these juvenile wood wraiths to my ends? Even though they were only mindless predators, all of natures' beings deserve to be free. Instead, I have made them slaves to my eternal quest — another act for which I must someday pay."— The Crazed Man(src)
Wood Specters are fast and agile, usually coming in small groups. Their melee attacks deal average Physical damage. One can detect their presence by distinguishable evil chuckles.
They can also spawn from hollow vertical trunks of trees if player gets nearby. Such a tree will be immediately struck by a lightning when approached, releasing 3 Burning Wood Specters, which are of red color and deal additional Fire damage with their blows.