Tucked away at the back of different versions of Creatures & Treasures are some interesting little add on chapters. In the first C&T that I owned it has the comversion stats for D&D and Runequest. In the RMC Creatures and Treasures it has guidelines for creating your own monsters.
I am a dab hand at D&D monster conversions as I convert from old FR modules to RM all the time but creating new monsters is not something I have ever done.
There are three immediate uses I can think of for new monsters but I only want to discuss one of them here and now.