Hello! Here's a puppy corral while we wrangle up an answer for you.

Please make sure to register Bruno's assistance with extermination under the 'critter' header below. Rémi will register his help and Remember This Later.

As for the thessyx, they will indeed confirm that 'leviathan' is the descriptor they use for the giant dead creature, and they seem to believe that it laid those eggs. We've answered a few questions about their society here, but we can add that their finn color is associated with their role. E.g., the three with blue fins are 'representatives.'