Monster (The) was a character played by Mark Steger in multiple episodes.

The directors, the Duffer Brothers, have this to say about the nature of the Monster's portals:

We don’t want to reveal much, other than to say it is an interdimensional being that has more in common with the shark from Jaws than Pennywise from It. When the monster enters our dimension, it’s like a shark breaching the water. Very much like a shark, it drags its prey back into its home, where it feeds. Each time it enters our world, it leaves a small tear, or wound. That’s what Joyce found in her wall in episode 4, and Nancy found in the tree in episode 5. These tears are almost like portholes into the Upside Down. But they don’t last very long. Like wounds, they eventually heal and seal up.
Entertainment Weekly Interview, July 20, 2016

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