I would like to suggest several magical creatures that could be in this world. Dangerous to men, mainly, and much less so to dragons, for obvious reasons:
Centaur: Wild half-men half-horse that are male only. They are powerful in nature magic, and generally do not speak or communicate with anyone but each other. Women are in peril if they see a wild one nearby- they need someone to make more, after all. Powdered centaurs hooves has a variety of alchemical uses.
Unicorn: Wild creatures, again, with lots of nature magic, coming in any color possible, with a differently colored horn. These are less sentient than centaurs, and they will only let women approach- men will be skewered. Their horns, if harvested, can be a powerful antidote for almost any poison or disease, should you dissolve a few shavings on unicorn horn in some water
Mimic: A creature that everyone fears, a Mimic is a floating column of bright, magical mist. The Mimic is named so because, when it hunts. it consumes the life and energy force of the creature, dissolves it into nothing, and then takes the same form, and is virtually impossible to tell from the real creature because it retains the same memories and mentality. The weaker the creature, the less the Mimic stays as it. A powerful magician will feed a Mimic for a dozen years, a regular human for three, and regular animals less than that. In reality, the Mimic is actually an extremely complex weaves of spells that, in many places, resembles necromancy. Not even a dragon can kill this one- arrows, swords, magic, it'll all go straight through the Mimic.
Cockatrice: It's essentially a two-legged dragon (like a wyvern) except with chicken feet and a chicken head. It breathes fire. Humans have extreme difficulty killing these; their magical aura makes it hard for spells to damage it, and its scales and feathers are very strong, like reinforced steel. The only one people have defeated Cockatrices is by drowning them, or having a dozen mages surround it and combine their forces.\
Gorgons: These are a twisted race of humans that have sprouted skin that, when you look at it, appears to have scales under the skin, with a slightly dry skin feel, and the ability to turn people to stone. People turned to stone do not immediately die- Their mind is still there, separate from their body, and if a magician turns them back soon enough (within a couple weeks, cause it takes like three or four days to undo a Gorgon's stone trasfiguration), then the stoned person will still be alive.
Death Viper: A small, 2-foot-long at the most snake, with bright red, orange, purple, black, and yellow diamondback patterns. The reason why you can see it from half a mile away is because it's the most deadliest snake in the world- its skin oozes a poison that, if you were to pick it up, would probably take your hand off. If it decided it didn't like you, and bit you, you're pretty much dead unless it bites a hand or foot, and you cut it off in a couple seconds after bitten. This creature can be taken as a Familiar (using a spell, you may have the chance to share a emotional/mental connection) and will not harm the person that has this snake as a Familiar- at all, not even the skin-toxin. These are dangerous, even to dragons (if it gets through the scale).
Wyverns: Unintelligent two-legged dragons. They don't grow as big as dragons and they breath a poison gas.
Trolls: Large humanoids that stand about 2 meters tall, and usually sleep out in the open because when they bend down, put their head to the ground and cover up, it appears as if they are a rock. An example of what they might do is to 'build' a bridge over a stream, and then wait in the stream for someone to come by. When they're in the middle, the bridge will usually collapse, into the troll's hungry teeth.
Nightstalker: These are invisible creatures, similar to a lion in size, that have next to nothing known about them. Besides the fact that they have wicked claws and Nightstalker teeth are occasionally found when ripped apart by a troll or other creature, it's really unknown what these things are. They only come out during the night- it could be that the sun reveals them, or hurts them.
I'll think up some more later.