If your players know what the tentacled panther is or immediately pulls out the torches when you describe the troll it may be time to start reskinning your monsters. Reskinning is the art of using a pre-existing monster stat block to describe a new monster with minimal or no changes.
I do this for several reasons.
It allows me to get more milage out of my monster manuals. Players get bored of fighting the same stuff over and over again, but by simply changing the description of the monster you can reuse monsters without them getting stale. You can also mitigate meta gaming with this tool. Everyone knows that fire and acid inhibit the trolls regeneration ability, but if the troll was a giant immortal crab instead of an ugly giant I bet it will take them a bit longer to figure out the creature’s weakness. It also has saved me countless times when the player’s go off the rails. If the players decide to jump down into a well and the only only hurdle you can think to throw at them them is ‘well gremlins’ you can just grab a semi appropriate monster and just call it that.
So how do you reskin a monster?
You can either have a bizarre creature you want to make or have a stat block you want to use but want to flavor differently. If you have a bizarre creature in mind, lets say a grey skinned mind controlling alien with tentacles, you just need to find a creature with similar abilities within the level range your looking for. So for this alien, lets say I’m looking for a CR between 4-6. A succubus has telepathy, mind control and deals psychic damage. It also can travel to and from the astral plain! It would make a great alien. The lamia is also great with charm person, geas, and intoxicating touch. A Night Hag with its dream haunting abilities would also be a fun take on telepathy. I just decide which one is the most fun option change its creature type to aberration, make whatever melee weapon attack it has deal bludgeoning damage to make them tentacles, and your done!
If your going the other direction it can be an equally fun process. Lets say you want to use the Umber Hulk in your next combat, but you’ve used them four times already and want to change it up. The easiest way to do this is to change it to another type of animal that is wildly different. The Umber Hulk is a bipedal insect like creature. I could make it a owl with a psychically confusing hoot, or even a scintillating anemone that has acidic pseudopods instead of slashing claws and a swim speed instead of a burrow speed.
Once you start reskinning your monsters you’ll quickly realize just how endless your creature choices become. Hopefully this helped demystify a bit of the process and inspires you to look at the monsters in your manual again with a new perspective.