Chimaeras are genetically engineered alien creatures, that burst into the diner, looking for Scott's Star-Forger Device. There are two of them in these scene, but they pretty much sound identical which is why I'm making it one role. 

In terms of appearance, imagine a creature with spider-like appendages, scales, sharp teeth and a face with no eyes if that helps you do the voice better. The closest thing to their design are the monsters from the movie A Quiet Place (the thumbnail image), but this is just a placeholder and I will be coming up with a new design.

They are mindless and don't talk, so you should be able to make convincing alien/monster noises.