The Unknown Tentacle Demon is a new creature featured in the DOOM Eternal teaser trailer
Near the start of the trailer, there is a small worm-like tentacle with spikes on the front of what might be it's face, possibly teeth. There are also spikes on it's chest. it is unknown whether it is a different creature or a variation of the Unknown Tentacle Demon. The creature may be able to fly as it floats/slithers off to the top of the screen. Later in the trailer, it can be seen wrapped around a support pillar, it crushes the pillar like a snake.
There are also Smaller ones as shown here that are shown in the new E3 Doom eternal demo, they have very low Health.
There are several larger Unknown Tentacle Demons wrapped around a bulding, the tentacles appear to be coming from one large mass, so the full creature may resemble a squid. The tentacles move around and destroy parts of the building as they move.