Why must pretty things kill me?
My current "pretty but painful" fixation is the blue sea dragon. I mean, just look at it.
Just look at that brilliant blue hue.
The blue sea dragon (glaucus atlanticus) is a sea slug found in tropical and sub tropical waters. They float upside down in the ocean, and that's also the reason for their bright blue colour. It is so that they can blend in with the blue of the ocean.
But my favorite thing about these creatures is the fact they are absolutely tiny. They only grow to be 3 cm. They are tiny little guys. (つ≧▽≦)つ
If they are just cute little guys, then what makes them so dangerous? Well, it's their prey. They feed on the portuguese man o' war (that's is a ridiculous name). The man o' war are like jellyfish without actually being a jellyfish (taxonomy is weird). The only thing notable about them here is that they have venomous nematocysts capable of delivering stings painful enough to kill fish and occasionally humans too.
Which, since the blue sea dragons eat them, gives them access to these stinging cells, which they store in their own tissues as a defense against predators.
Because these slugs concentrate the venom, they are capable of producing a more painful and deadly sting than their prey. Which shatters my dreams of petting them because I will die a miserable death.