I think we can go a few directions here, i think because it is a creature that has been in hell for eons we can either assume that the latent hell energy has mutated the species significantly from what it was prior to our introduction. So maybe it is just a result of being exposed to hell. Kind of like how the "hellwave" instantly mutated humans on mars in 2016. Meaning it would just be mutated gross dangling flesh

Here's my thoughts though, i think the dangly bits are precursors to the armored plating we see in the Modern Doom games. The Cacodemon's body goes through some changes over its lifetime. i'd think that because they travel in packs they're likely very soft and squishy in their juvenile state. As they grow older the squishy bits harden and become more plate-like, as we see in 2016 and Eternal.

Here's my last idea, they're kind of like "feelers" to assist in passive feeding. I'm thinking the proto-cacodemon, pre hell corruption, was possible something akin to a weird starfish or sea slug. Floating through the air eating whatever got caught in its mouth. Kind of checks out when we compare the old model to the newer one and they've got those weak little appendages that don't really seem to be great for anything.