The Cave Spirit is the main antagonist of the Give Yourself Goosebumps book, The Curse of the Cave Creatures.
The Cave Spirit will either appear as an animal or a human magician depending on which storyling you choose - If you decide to face him as a hunter, he will take the form of various animals, which will lead you facing his true form - a bear, or if you decide to face him as a spellcaster, you’ll have to solve many magical puzzles before facing his magician form.
The Cave Spirit has the ability to make any animal (even toys and pictures) aggressive and dangerous towards the reader.