This Monster Has No Picture

If you doubt the value of art, look upon the creatures in the AD&D Monster Manual that have no illustration. When you leave out the "you know what they look like" (bears, lions, etc.) and the hard to see vermin (cerebral parasites, ear seekers, floating eye, slithering tracker, gelatinous cube), and the "souped-up version" (ghasts but tell me why, why the neo-otyugh gets two illustrations of its own) and the inexplicably passed-up opportunity to illustrate a nymph ...

and the masher, already humiliated enough from losing the purple-worm status it enjoyed in Blackmoor (but it had art, it just got lost) ...

you are left with the monsters that have no illustration because there is nothing to illustrate. There is the shadow, which is visible but shadowy and also would have made a fine illustration. And then there are the monsters that are invisible by nature.

Also inexplicably, there are three of these. You have the aerial servant, the invisible stalker, and the wind walker. Each of them is spun off from the air elemental with a few variations in immunity.

Ral Partha's Aerial Servant. Funny.

Does nature create redundancies?  Ask the dolphin, tuna and penguin. But art demands unity; novels reuse characters, though it stretches the credibility of coincidence; and so, there is something more satisfying about surmising that these three creatures are all just different aspects of the same elemental being.

Otherworld's Invisible Stalker. Funny.

• The aerial servant responds to a cleric's high-level spell. With the wisdom of holy magic, the spell only contracts the servant for a clearly defined short-term mission of capture and return, using the power of religion to control it. As a result, the creature serves willingly and commits more of its essence to the task, appearing with a nigh-unstoppable 16 hit dice.
• The invisible stalker responds to a magic-user's high-level spell. This contract of service is more loosely defined, and tempts the wizard to push his or her luck at the risk of having the stalker undermine the instructions. Under these terms, the creature commits less essence, appearing with only 8 hit dice.
• The wind walker is one of these creatures, loosed from service by the death of its summoner a long time ago, or bound to this plane because of some mishap. It is easier to hit because it abandons its dutiful silence to howl and rush, and weaker because of its long residence on the material plane (implying that there are fresher, larger specimens roaming around.) Alienation from their homeland has also turned wind walkers evil-natured and indiscriminate in their destruction. Their telepathy is something implicit in the description of the aerial servant and invisible stalker, else how would those creatures understand instructions in all the possible languages of wizard-dom?

Wind-powered walker is much cooler, anyway.

So, although naturalism in a magic world may breed a huge variety of invisible monsters, from the players' point of view the whole game of figuring out "oh, what invisible monster is attacking us today,or maybe it's a normal monster turned invisible by that darn mad wizard" is not really worth the candle. As well as being disrupted, very simply, by the handy bag of flour.