The Fundamentals of Writing Monsters

There's a lot about some of these monsters that I feel misses the point.  If we look at a basic description of something like, say, the displacer beast, first we get a picture, then we get a description of the picture, which is really nothing more than filler.  Usually, the description doesn't even match the picture (the image does not show the tentacles sprouting from the creature's shoulders, but from the middle of its back, in a very poor representation).

Then we read about some special ability, "the displacement" feature, in terms of why the ability exists ("light-bending illusion") which does not actively say what happens if you try to compensate for it or bother to describe the effects of hurled missiles at said illusion.  The original monster manual (not the linked site) gives the displacer beast a -2 AC for the ability, which is paltry to say the least, and completely boring as shit to say what's deserved.

Then we get some meaningless vague references to the beast's ancestors, the attempts to breed the beast (with no rules or mention of the players doing this themselves), a list of creatures the beast could supposedly be bred to kill (I'm sure a cow and a horse would also work) and an argument that you can't breed the beast because it is too evil.  I have no idea how they're able to "use their malevolent intelligence to escape their masters" because this isn't explained; the beast doesn't phase, after all, it is a light-bending illusion, so what? The masters feel intellectually required to leave the cage door open?  We're not told. Shut up, accept this as dogma and move on.

Then we get a short account of why displacer beasts and blink dogs hate each other.  How is this useful?  Are there going to be a lot of adventures where it will be really important to parties to not have blink dogs and displacer beasts attacking each other?  Seems to me, humans and displacer beasts will attack each other on sight too, without the need for Seelies.  Many creatures everywhere will attack one another on sight. So what?  How am I getting information on making an adventure if I choose to throw one of these things at a party?

claws and teeth only for show

This is the problem overall with monster manuals as I read them.  They are giving the wrong information.  When I'm writing these beasts, I'm trying to establish three things: where are they found; what can they do; why would they interact with a party?  Being found is generally easy; I'm putting them in some vegetation and climate, somewhere on the Earth.  What they can do requires a little more than just saying they have tentacles: how far can the tentacles swing, how does the displacer beast use them ~ and if the beast is also a cat, why doesn't it also have cat-like attacks?  

My monster manual just gives 2 attacks for the tentacles and ignores the big teeth and claws that are depicted on the old displacer beast image. How does that make sense?

Too, I want something more meaningful for the displacer beast's "displacement" ability than a light show.  Really? That's as imaginative as we can get?

Finally, I need some kind of rational written into the monster for what would happen if a party might encounter the creature.  Would the creature attack them, and if so, how?  By what method?  At what point might the creature run away.

One thing I like about the wiki, if I think of something new, if something comes up in the middle of a game, I can go and fix the page immediately, upgrading it as necessary.  Putting it on the internet might (conceivably) have someone point out a flaw or a context that can add to any given monster that's been written.  It is an ongoing process, rather than a stale rewrite of a rewrite of a rewrite, which is what the fifty versions of displacer beast are that you can find online since its first depiction in 1979.  If there are more answers to those three purposes ~ where, what, why ~ it can be added later.

If you want to read my take on the displacer beast, you can find it on the wiki.