This shows Sophia Lillis' character transforming from a horse to the much-more powerful beast, the Owlbear. However, it's worth noting that the Wild Shape ability -- the class feature that allows druids to change into animals -- wouldn't allow a transformation into an Owlbear, as it's technically classified as a monstrosity. This could mean the filmmakers are playing with the combat rules a bit, or a spellcaster simply cast Polymorph on her.