Right now, we have a healthy assortment of microscopic species to murder interact with. Most of them, like the feeble Teeny and the sessile Algae, are completely harmless. However, there are two protists that aren't so harmless.
The veteran of the two, existing since version 0.2.4 (I believe), is Toxin Predator. Alone, it will, like the harmless (and motile) species, aimlessly wander. However, once the player approaches, it, as the name implies, makes use of its toxin vacuole to kill the player. Indeed, it will ceaselessly hunt you down until you move out of rendering range. They're fairly slow, with only two flagella, but then again there's plenty of compound emitters and other cells to block your escape path.
The second is a newcomer, Glutony. Looking at it, it seems like a benign, gentle giant. But wait, there's one feature they added... And then you realize that this thing is in to showcase engulfment mechanics. With its trawl of vacuoles, it uses the surface area to consume every living being in its path. Like the Toxin Predator, it will chase you without end. In terms of carrying this out, it's the opposite of its smaller kin: Glutony has a whopping four flagella (that's twice Toxin Predator's), but its large size means it is easily slowed down by obstacles.
Both of them are persistent predators, each using one of the game's two means of combat. Both show terrifying persistence in the hunt (if the Behavior Editor were in, I believe their trait would be "IsSlenderman = true").
Toxin Predator. You can evolve to the point where Gluttony is irrelevant, but Toxin Predator will always be able to hurt you. The worst part is that you can't even really kill it back, because that would just release all its OxyToxy at once in a terrifying last attack.
The gluttony isn't too dangerous, but they get annoying pushing you around even if they cannot eat you. The poisonous predator is easy for me to handle because I can easily push them to a desert place with no nutrients or a place with no oxygen. I picked Gluttony mostly from it's nuisance.
After a pretty short time (maybe also because I played it the whole day once), you can even eat the Gluttony. You just press G and they just give themselves to you. Toxin, on the other hand, will always be able to do damage...