One of the things I noticed in RPGSS is that monsters and magic items have many interesting parallels. An item or a monster is basically a bunch of abilities wrapped in flavor. For this reason, a lot of the advice for wondrous items also applies to monsters.
Below are a few things I consider "do's and don'ts" in monster design. These bits of advice are actually derived from Sean K Reynolds's "auto-reject" advice; I just re-purposed them for monsters. The parallels are not limited to just SKR's advice; try and see if you can think of any advice you've found useful in wondrous item design, then try to apply it to monster design.
- A (Superstar) monster should have at least one unique ability. Creatures with only universal monster abilities and/or spell-like abilities often appear in bestiaries, but to show what you really can do, it's a good idea to give the creature a completely new ability or two. A creature that only re-uses other monster's abilities is the monster counterpart of a "spell in a can". (cf. SKR #1)
- The abilities of a monster should make sense when you consider its ecology and habitat (and culture if it's intelligent). Why does it have acid resistance? Does it live in an acidic environment? Why did it develop that defense? A monster that has a random collection of abilities just because they're convenient is similar to a "swiss army knife". (cf. SKR #2)
- A monster should not be just a plot device. If its abilities aren't any good except in a very specific situation, you may have fallen into this trap. (cf. SKR #4)
- An original monster shows much more innovation and mojo than a spinoff/variant creature. (cf. SKR #6)
- Monsters are allowed to have some backstory. For example, an undead creature's entry often explains how a creature turns into said undead. (cf. SKR #3)
- Monsters are allowed to be gross because they're your opponents, not something you have to wear. (cf. SKR #19)