In an answer to another question I made the point that using non-standard variants of published monsters has been common practice since the early days of D&D. This was based on my own experience, but I am certain I have seen the practice in published aventures. What is the earliest instance of a variant monster in a published D&D adventure?
How I am defining the term “variant monster”:
- A variant monster must be based on a published, official monster but differs from the official monster in a significant way. By significant way, I mean any change in physical statisics (including hit points outside the range normally possible) or a change in the monster’s physical description that might cause players to misidentify it or not notice it (example: red slime with green slime stats).
- Includes any monster with abilities not accounted for in its official description, such as a spell-casting medusa, a psionic basilisk, a day-walking vampire, or a water-breathing owl-bear.
- Includes any monster that behaves in a way that would normally be impossible for that monster (example: a sentient iron golem that acts on its own free will). But this does not include a creature that has been turned into a monster and still behaves as its natural self (example: a gnome trapped inside an iron golem’s body).
- Does not include new monsters that are not based on existing oficial monsters.
- Does not include new types of old monsters that receive a full description in the module and possibly later were published as monsters in their own right in supplements. Example: the drow (mentioned briefly, not statted in the 1e Monster Manual) was fully described and statted in the appendix of the first adventure in which they appeared (G3 Hall of the Fire Giant King) and was later published in the 1e Fiend Folio, so drow is a unique monster in its own right, not a variant monster.
- Does not include monsters that are physically and statistically the same as their official type but behave in an unusual way (e.g., a good-aligned red dragon or a cunning, educated ogre).
- Does not include monsters equipped in an unusual way.
- Does not include variants that are actually mentioned in the monster’s official description. Example: the 1E AD&D Monster Manual description of the ghoul mentioned the existence of lacedons, an aquatic ghoul that conforms in all other respects to a standard ghoul. The manual does not give a swim speed for the lacedon, which presumably it has, so giving a lacedon a swim speed in a published adventure would not count as a variant, unless other of its stats or characteristics have been changed.
NOTE: The question specifies D&D, which includes all editions and Pathfinder. Published adventures would be any official adventure modules, adventures published in Dragon or Dungeon magazines, or third party modules designed for use with a qualifying D&D or Pathfinder system.