Many of the games I have played are based around military forces fighting other military forces. This goes for fantasy settings, near-modern settings, and sci-fi settings.
I've used (and seen) monsters mostly in fantasy settings. The idea of a bunch of beasts joining together makes sense when you have orcs and elves running around. I think they can work in sci-fi settings as well. Starship Troopers for example. Many of the G.A.S.L.I.G.H.T. games I have played have made use of individual monsters that don't necessarily belong to either side and just add an element of unpredictability.
I painted up some spiders for this purpose. The models are old GW goblin spider riders and Warhammer Quest spiders.
These have been used for Lord of the Rings games, but I think they will be seeing the field of battle in some other games as well. These are great because they can be used in 6mm, 15mm, or 28mm games. Their power can be adjusted to reflect the scale. In a 6mm game these guys are monstrous and will tear apart tanks. In 28mm these guys won't do a lot to a tank by themselves but will be a challenge for infantry if they get into close combat.
I did not paint most of the colorful spiders. They were part of a purchase of used miniatures at some point.
Do you use monsters in your games? Is it whole armies of monsters or just a few hear and there?