This is another in my series of posts looking at Trick Monsters - the ones that use some kind of interesting or surprising change that makes them a bit different than a straightforward monster encounter.
Some monsters look like other monsters.
The look-alike monster has the superficial appearance of another monster, either more dangerous or less dangerous. The goal here is to confuse the players into either underestimating a monster (and thus being taken by surprise by the threat) or overestimating the monster (and wasting resources or falling into some trap).
Usually the look-alike is paired with some other awful trait punish you for trying a tried-and-true tactic against a different monster. Sure, mummies are vulnerable to fire, but these mummies are fireproof (or worse, burn like torches for hours while they grapple you.) This looks like a normal skeleton, but it's a lich. Or even, it looks like a chest, floor, or ceiling, until you get a bit too close . . .
This is a bit different from a merely upgraded monster - a leader-type orc or a more powerful ghost or the troll king. Those usually have some defining trait to make them clearly a different monster. What I'm talking here is ones that are functionally identical; where what you see isn't what you get, and clues are an after-the-fact thing.
Gas Spore - looks like a Beholder, actually a flying bomb. This is a trick in two ways - if you engage it as if it was a beholder, you're probably wasting resources blasting it with everything you've got right the get-go. If you don't, you risk it getting close in and doing bad things to you. And either way, if it's too close when you successfully engage it, it will explode and possibly kill you.
Nilbog - Totally unfair monsters from the awesome-filled AD&D Fiend Folio. They look like goblins, but you can't kill them by hitting them. Oh, and you could end up losing your treasure from encountering them, too.
Shapeshifters - pretty much any shapeshifter monster falls under this, either because it's acting like it's not a monster (the Choke Brothers, Throttlers from my own game), or because it's taken the appearance of something weaker with a weakness the shapeshifter lacks (a werewolf in wolf form, say, unkillable without silver).
Reeks - from Yrth, these all look the same, but their powers range wildly from mildly caustic up to spell-casting (or possibly psionic.)
I'm not aware of any variations per se - once it doesn't look like the original monster it's not a look-alike. You do get a wide variation in what is different, though.
It's a Trap! - The exploding monsters, sticky versions of monsters, etc - the ones that make it bad to kill them.
I love silver! - some have different vulnerabilities. Looks like a werewolf, but it's not, so silver doesn't affect it - you need something else to bother it. Looks like a demon, but it isn't, so all that holy symbol waving gets you no where.
I come in peace! - friendly versions of monsters are there to show the cost to shoot-first policies. You know, the whole "we look like trolls, but we're peaceful gift-bearing travelers from the dimension of ----aaagh! Not fire! Aaaaa----what we could have taught you . . . " bit.
I'm a big fan of trick monsters, but I confess I use relatively few of these. Generally I prefer to give a clue - even a weak clue - that the monster is different. Or just use a different monster. But hey, gas spores. And I'm not saying my reeks can't be spellcasters or telepathic death slimes, just because they all look alike . . .