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 RPGs. 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 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 or a psionic basilisk.
- 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 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 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.