The 'Same but Different' Approach

 "An eight foot tall, green, slouching monster steps out of the swamps..." - GM
"It's a troll everyone get ready!" - Player 1
"It only has 125 HP and 20 AC. It should be easy for us. Plus we have fire and acid. Oh! Stay out of close range because of its triggered ability!" - Seasoned Player 2

Wiosna (Spring) by Michalina Janoszanka, ca. 1920

If you've been playing TTRPG's for a while you've likely seen one of these kind of players. Maybe it's done to show off, or maybe with the intent to help, but in my experience these sorts of players can be a downer for GM's and often take other players out of the fiction; not to mention absolutely ruin any sort tension at the table. Luckily there are a few different approaches to solve this problem, one is the perennial Talk to the offending player, and hash it out strategy, which can highlight if it's an attitude problem and if so; tell 'em to take a hike and find someone better, you deserve good players! Another solution, and one that I utilize often is the Same but Different approach.

If you've taken classes on Creative Writing before you made have heard this saying before but to summarize: When you are writing genre fiction and you want to add a number of original ideas, you need to balance them with 'Normal' things within that genre so that readers (or players) can still feel grounded in the fiction. Think of it like this, if in a D&D game every class was something you've never heard of, every ancestry was completely original (Multi armed slime from outer space!), and even every weapon was named and described as something you aren't familiar with it would be VERY overwhelming to try and make a character and during play it would be even worse! 

"Drak uses his Avader class ability to conjure forth the Atium and throw his Rutul at the Borgesh." Sheesh, what even happened there?

As for what that balance is? Usually they recommendation is 70% 'Normal' and 30% 'Original' but this can fluctuate a bit.

Now you might be saying "That's how a lot of games sound when you first start playing!" and you would be right, however many games also ground the players with things most people ARE familiar with. Even if you don't know TTRPG's you likely know what a sword is, or a Wizard, or (maybe) even an Orc. These concepts are either grounded in real life or are ideas that have escaped containment from Fantasy and therefore can act as something grounding and recognizable for the players. The amount of 'Original' ideas you add in can also increase as time within the fiction increases as players (or readers) become more accustomed to the world and adjust to the fiction that you are telling. Okay with that being said, how does this relate to the 'Seasoned player' issue?

Fantasy of Ruins with a Statue of Minerva in the Center Foreground by Giovanni Battista Piranesi, ca. 1748

One of the best ways to implement the Same but Different approach is to give your fiction a Fresh Coat of Paint. Even simple goblins can become deeply mysterious and strange when you describe them as "Four armed, bat eared, and covered in thick curly black or brown fur." You can keep their stats completely the same and the players wont be able to tell you what these are unless you tell them. Another implementation is the Engine Swap, take a monster that your players are likely to know and swap it's abilities with a similar creature.† Imagine if the above Troll scenario was followed up by this description:

"As you approach the Troll belches a gout of flame, catching the ground in front of it ablaze. Thick black tar leaks from lesions on its flesh, the pungent scent of burning oil permeates the area." - GM

There is nothing quite like having a set of seasoned players waltz up to a troll and being overconfident only to be put on their back foot when it begins breathing fire. You can get a lot of mileage out of these two approaches and I often find myself using these in every campaign I run, but remember not to overdue it! Having typical monsters and predictable enemies releases the tension that these new Same but Different monsters cause, and makes it more impactful when one does show up. If you feel that this still is not enough feel free to do a Custom Build and make your own creature wholesale!

And remember, if you can't think of what monster cover in a Fresh Coat of Paint the handy dandy BEAR will always due the trick.

†. Try to stay in the same level range as the base monster unless you're well versed in your system of choice and how its balance works.