If you didn’t read the New Mutants comics, you might not know that Demon Bear made its debut in New Mutants #3 (released in May 1983), and as you might expect from the name, it’s a giant bear creature who derives the bulk of its power from negative energies.
In the comics, he looks like this, though you can click over to ComingSoon for an idea of what Demon Bear will look like in the New Mutants movie.
ComingSoon is reporting that the villain Demon Bear will be the main antagonist in Josh Boone’s planned series launcher, though the movie doesn’t yet have its own confirmed release date.