Well, it's a Pokémon game so most likely that fluid isn't something we can find in real life, nevertheless we can speculate what is its purpose. If you watch closely you'll be able to see "bubbles" floating in those "muscles" so I can safely assume that the liquid isn't inside its body or it die due to bubbles of gas or small amounts of lighter fluid in its blood stream, I haven't done much research but since buzzwole live in a jungle-like environment I would assume that fluid is some kind of lymphatic system that buzzwole use to get rid of some kind of dangerous gasses, if those are bubbles, or byproduct of physical activity, like lactic acid if those are other kind of fluids, and being transparent help buzzwole prevent sunburn because the uv rays are absorbed by that fluid and not directly by the skin.
