Never to be encountered in my world!?!
One of the ‘conceits’ of my campaign world, is that the dominant integral species are insectoid. But due to my many years of management experience and an addiction to efficiency, I’m a firm believer in not re-inventing the wheel. The GW boys made a damn good game with a very well thought out bestiary, so it doesn’t take much skimming to find a creature/humanoid that has the stat-line and special rules to something I’ve dreamed up, and only the description will need a quick re-skinning (and maybe a quick sketch) and my work is done.
A recent example was when the party had stumbled across an excavated starship that had, centuries ago, popped out of hyperspace inside the planet. Every space inside the craft was covered in Hypergeometric runes, so they had an idea something was awry. For the pilot and copilot I wanted a tough undead-sort, so I grabbed a couple of mummies, shaved off one of their attacks, and proceeded to watch the reaction of my players, as the blasphemy-mumbling crew members absorbed a couple of fireballs (reskinned into the ‘rending’ spell, just can’t help myself) and kept on fighting. Nothing too inventive, but it made for an exciting encounter.
A minimum of work with a high entertainment yield. Well, for me anyway.
So my players can expect to never encounter a dragon in this campaign, but they can bet their asses they better be careful of running into a massive, flying, fear/terror-causing, 6 attack, venom-spewing bug.