These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: Grays. These appear in the not-so-rare instance where an Oozle absorbs a gloople. Grays cannot be damaged until they try to impale you with a flexible spike, at which they soften up and you have a short time to hit their body. And you've got to do it three times in order to kill it.
Pretty much all of the more dangerous glooples can be considered this, as they appear very often in larger nests and can be pretty nasty. Torchies, in particular, will explode if you try to attack them while they are still on fire, or light you on fire on contact.
Goddamned Bats: Biters fit this to trope to a T. They appear in numbers, actively chase down the player, move at a fast speed, and utilize a fast lunge attack when the player is in line with its face. Which will kill you instantly, as you're a One-Hit-Point Wonder. More often than not, these guys will actually get you. Oh, and if you let two of them bump together, there's a very small chance that they'll merge into a horrible Demonic Spider.
The awards section even tells you "These little guys still kill you more than anything else don't they?"
We also have Fuzzles, a modified Biter that's basically a Tribble from hell; they take three hits to kill, have a potent Healing Factor, and can lunge even farther and faster than Biters can (though they take a while to get ready). Fortunately, they can't merge into their own Demonic Spider. Also, they try to run away from you when they're one hit away from death, which is a good opportunity to get a breather (or to just finish them off.)
Clutters. Oh boy, clutters. They fire out baby clutters which stick to you and slow you down to the point where you cannot move, leaving you open to other nastier blobs. You have to spin around in circles to shake off the baby clutters (which will grow to more adults if kept on-screen).
Thankfully, baby Clutters will not stick onto you if you have the Hazmat suit. And you can chase Clutters (baby and adults) offscreen, effectively dealing with them. (You also get an award for doing so)
Melties can appear in numbers and try to home onto you. It doesn't help that their acid is one of the rare things your Reactive Armor can't save you from. Occasionally, their fragility causes them to burst on a nearby gloople, melting you instantly, despite the fact that it never touched you.
Goddamned Boss: Depending on when you encounter it, the Amalgam is either this or That One Boss. It's the latter if you encounter it in the middle of a nest with many Glooples on screen for it to devour, then it's nigh invincible. It is the former if you encounter it at the end of a nest while waiting for a Queen or a Razor Queen. Since you can't get rid of an Amalgam without killing it (it can't be lured offscreen like other Glooples), you'll have to kill it and put an end to your nest before you can encounter the enemies you need to kill. (Though items that don't give you kill credit, like the Reactive Armor, can help with that).
The Void Eater's disintegration beam, which sucks you in and kills you instantly, bypassing every possible defense you have. If you can't splatter it before it finishes charging, you might be in trouble (especially if something is impeding your ability to move).
The Horror's tooth volley, which can kill you AND create more Biters to pester you. Also the Horror's splitting move. Like the above beam it's the only opportunity to hurt the Horror, but it's even HARDER to avoid than said beam.
A Queen's charge attack.
The Razor Queen's entire projectile-launching stance can be considered as one of those. Notably within it, those dang homing razor mites that travel underground.
That One Boss: the Amalgam is very much this if you encounter it too early, since its ability to devour things and duplicate itself makes it almost impossible to destroy without usage of high explosives.
Everything and anything in the game that can be called a boss, is one of those. Many have devastating and hard to dodge moves and often require very precise tactics and skills to beat them. Also many appear right in the middle of a nest, meaning that they are assisted in slaughtering you by countless more Glooples.
Of course the Razor Queen exemplifies this. Just to give you an idea, she has no less than six nasty ways of killing you, her blades block here body for almost the entire duration of the fight & she takes up at least a 9th of the screen. She makes all of the other enemies look like cake-walks by comparison. Just be glad that without a glitch, only one appears onscreen at a time.