Monsters will be some combination of dangerous and weird. Many bear at least some resemblance to creatures from myth and folklore. Most creatures from fantasy RPGs should be appropriate, though not all. At their simplest, monsters are animalistic, like a massive boar or snake. At other times, they will be bizarre amalgams. They are often malicious and never safe. Intelligent monsters can act benignly at times but are always alien never sane from a human perspective.