At Dungeon Fantastic and Gothridge Manor Peter and Tim talk about on player knowledge of monsters. Generally I am not concerned about it, run a specific RPG, and/or campaign multiple times your players will know the mechanical details of creatures, items, and campaign secrets.
So how to keep it fresh? Many would advise to change things up with the creature itself. Maybe this Medusa doesn't turn people to stone but instead charms them in order to kiss them while the snakes in her hair feeds on the unfortunate PC. Done right this can led to a memorable adventure and a highlight of the campaign. But done too often it can get tedious as the referee comes off capricious and arbitrary.
Make new creatures? Same issue, it can be memorable once a while but done too often your campaign feel like the menagerie of the weird.
Because I chose to stick with one setting, the Majestic Wilderlands, for my fantasy campaign I was confronted with this problem early on. It ceased to be an issue for me when I realized that you can think of the Wilderlands as a living breathing world. Walk the street of any town on a busy day you will see a dozen or more people around you. Each with a unique story and motivations, all human. The same variety is seen when biologists track animals in the wild. There is no reason why orcs, elves, dragons, monsters can't have the same variety.
Of course we are not Commander Data capable of simulating the life of planet with digital timing. So the referee have to be creative in how introduce variety. For a specific creature or fantasy race, you think of the broad possibilities. While people and creature are individual, when looked at in aggregate the average is shaped by environment, and history. There are always outliers but individuals tend to somewhere fall in a broad range. So the first thing is to figure out what is that broad range for a fantasy race or creature. Then think of the variations within that range. About a half dozen should do.
You will find in coming up with that list you will see connection and possibilities not only for a adventure but a multitude adventures. Plus you will add to your bag of stuff so that when the players decide to go left into Orc Forest instead of right into the Mountains of the Moon, you can quickly throw together an Orc Adventure.
Start you off try to think of how orcs would actually be if they lived in your setting. Live, eat, make stuff, play, love and yes make war. Then move on to the monsters and race that dominate your campaign. Remember you don't need a lot of detail, If you end up with a page of notes you done enough as long it provides the spark for future adventures.
This technique uses roleplaying to overcome the limitations of game mechanics. In this case knowing what the creatures stats are.