How did it even fit inside?
A horsehair worm (Nematomorpha) emerging from its host.

Inside the host, the larva grows, developing into a full adult. As development nears its end, the worm begins manipulating its host’s behavior, driving it to seek out any nearby body of water. Once there, the worm forces its way out, burrowing through the host’s tissues—leaving the host to die.

Animazing