aco175
Legend
I tend to have the base monster plus modified versions. Like what @Oofta said above, if the PCs ran into base gnolls before, then tell them there is one that looks more like a caster and another wearing more armor and looks like he has been in many battles. This allows the players to expect something a bit different on some or all the gnolls.
If you want to make CR5 the base for gnolls in your world you can, but maybe call them something else. Over time playing, the players learn certain things about the world and the game. Your game and world might be different than mine, but we have things that work the same. Is an orc and orc or a troll a troll. Then ask what would someone who grew up in said world would know about monsters, what about an adventurer who is going out to fight them. This is why I tend to keep the base monster so everyone playing can know trolls need fire to kill them. They can make up their mind when I tell them that this troll is different.
If you want to make CR5 the base for gnolls in your world you can, but maybe call them something else. Over time playing, the players learn certain things about the world and the game. Your game and world might be different than mine, but we have things that work the same. Is an orc and orc or a troll a troll. Then ask what would someone who grew up in said world would know about monsters, what about an adventurer who is going out to fight them. This is why I tend to keep the base monster so everyone playing can know trolls need fire to kill them. They can make up their mind when I tell them that this troll is different.