I've bought the Humble game making Bundle. It seemed a nice way to get some assets so maybe I can make a game for once, because I'm a horrible artist.

There are some resource packs for rpg-maker, and most of them include something called "Battlers". They seem to be static images of characters (mostly monsters), although googling about I have found that there are animated versions, too. They come in two versions: "side" and "front".

There are some examples: https://vxresource.wordpress.com/category/resources/battlers/

I'm curious now, what are them supposed to be used for?

All I can think is ultralow-budget "cinematics", or maybe an old school j-rpg combat system, with the PCs in one side and monster at the other side. The former could be acceptable (IIRC the intro for Legend of Grimrock was made with static images), but the later would be very dull without some attack animations.

Even the original Wasteland and Dragon Wars had some animations in the combat screens! (no attack animations, though)

They're used like avatars, not directly in combat.

Oh, that makes sense, I guess. I didn't though of that because the first set of battlers also included portraits for each creature, and they seem more apt as avatars to me.

Any well-known game that uses them?

You use them as avatars for the creatures too, if the creatures can control fights rather than just being a unit in them. They are commonly used for CCGs, RTSes, TBSes, and games where the player is inside a vehicle during combat or racing. The term Battlers probably comes from the way they are used in the older Pokemon games.

Here's a race choice screen which uses battler-type images:

Same game, reusing the images as avatars:

You yourself can also make color-rotations of the battlers to get more of them, if you want. And they can be faux-animated the same way any sprite can, like making it hop, wiggle, or flash.

Thank you very much, that was very illustrative.