unknown "thing"

[SPLiSH]

how do things like that get in tanks?

Through contaminated live food, plants, even some fish. It looks like some weird bug larvae thingy... maybe keep it in a bowl (no fish) until it grows up and see what it is then. But if you do keep it you should probably cover the bowl because it doesn't look like something you want flying around your house.

 

[CFC]

They are caddisfly nymphs, they are micro predators and will eat any fry or very small fish they can catch. Get them out and put them in a pond somewhere where they can live out their lives without hurting any of your fish.