They are vet masks. They are generic and come on different sizes. The largest one can fit several small animals. The smallest ones are for cats and tiny dogs. I think the largest one they make are for horses and cattle. But they have special fittings for straps. This ones are rubber ring ended so they are easier to use to makeshift a tiny gas chamber.
It’s so satisfying to see mangohud jumping into the hundreds of FPS while knowing the game is cranking the highest quality image fidelity it is capable of. It’s like taking an apartment dog to a park and releasing the leash. Run baby, run as fast as you can.