Named giant animal

My hunter got killed by Tollissedil Giant Mantis and i am wondering why this mantis is named? It is not a forgotten beast and after sending my military at it my dwarfs refuse to bring him to the slaughter house, is this because he is named? Might it be a pet from a invader?

Wild animals get named by a few means; a common one aside from being a pet is if they manage to kill enough enemies. You can check it in history mode for yourself - quit the game, copy the last save somewhere, restart and abandon the fort, and restart the world in the history viewing mode. Tolissedil will be in there, somewhere, with all of his exploits and killings. You can restore your old save with your functioning fort afterwards.

"Tolissedil" is a Dwarven name ("tolis" = top, "sedil" = angel), so chances are it only got that name from the killing of your hunter.

See also: Animal and Megabeast names on DF wiki

Besides being adopted as a owner animals can also be given names during world gen if they become an enemy of a civilization or if they have any notable kills.

They should, under normal circumstances, be butcherable so I don't know why that happened. It could be because the corpse is to damaged.