A Tyrant is a creature that mainly stays in the water. It looks like a giant crocodile.
Best way of doing this is with yourself and a friend on the Nest game mode and on the map Distillery. Find the Tyrant and walk to it to get eaten, then you need to shoot the Tyrant until it frees your friend.
I have included some screenshots to give you an idea of what the creature looks like and where you can find it on the Distillery map.
If there's anything else you think I could add please let me know.