Druids were first introduced in the Greyhawk supplement for Dungeons & Dragons as “priests of a neutral-type religion” with the ability to shape change into an animal three times a day (Gygax and Kuntz 1976:34). The class was fully fleshed out in Eldritch Wizardry, with nature-themed spells, the ability to pass through undergrowth, identify the elements of nature, and the power to change shape into an animal form. They had an obligation to protect nature, especially trees (Gygax and Blume 1976:2). In later versions of Dungeons & Dragons, druids evolved into neutral-aligned masters of nature and reincarnation, capable of communicating and controlling plants, animals, and even the elements themselves.
The term druid is originally from the Gaulish “druids” which came from the Old Celtic “derwijes.” Sources diverge as to the meaning of druid. “Derwos” means true, which would make the druid a truth- or soothsayer. Alternately, the Old Celtic base of “dru” means tree, in reference to the importance of oak trees in druidic ceremonies (Ayto 1990:186).
Historically, druids did revere trees as nature spirits. They had a healthy respect for the animal kingdom because any animal could be a god or another heroic Celt in animal form. Zoomorphism was common enough to justify druids transforming into all kinds of animals. But unlike their depiction in Dungeons & Dragons, druids were not exclusively animal protectors. Nor were humans exempt from this protection.
Conspicuously lacking from Dungeons & Dragons is the role of sacrifices in druidic rituals. Historically, druids sacrificed everything: bulls, dogs, stags, slaves, criminals, gold, silver—they burned, drowned, strangled, or examined its entrails. Druids would stab people in the back and divine the future based on how they twitched when they died. They believed in regicide, utilizing the fabled triple-death of strangling, drowning, and stabbing with a spear. They crammed gigantic wicker colossi full of people, if Caesar’s account is to be believed, and torched them in sacrifice.
Druids could be classified as neutral alignment, in that Druids were on both sides of morality. Celtic stories are filled with accounts of Druids on both sides, undoing each other’s spells, and battling in fantastic magical combats that would make any role-player envious. But the alignment of druids as being true neutral was simply not accurate; druids were just as passionate about one ethos as anybody else.
This description is a rough draft from my upcoming book, The Evolution of Fantasy Role-Playing Games.