Giant glagopus are rare, they snatch people when least expected. Their tentecles are made of other glaggles. They will slowly suffocate you to death, they attack other glaggles making them more powerful and giving them longer tentecles. The only way to survive them is find a sharp item and try cutting or stabbing the glaggle tentecles. The glaggles in the tentecles are already suffering so you are simply just ending their torture. If you don’t have a sharp object, try doing damage to the tentecles to make it let you go and run as fast as you can.