For the sake of example, this monster is old, sad and hungry.
Behind your screen or in your book, note these and nothing else. Beyond this, there is no monster.
Do not describe the monster other than through what it is; if the players ask about its size, for instance, speak only of how it has withered through age or starvation, or grown with its insatiable appetite. If they wonder about its appearance, consider the effects of its mood or its long life on its colour and form.
Your players may ask what it is - you only tell them it is old, sad and hungry. "No", they say, "what, it must be something", and list names of monsters they know, guessing. Their guess is as good as yours. All you know is the truth; it is three things. Anything they guess that does not contradict the truth may as well be treated as accurate, if only for the sake of manufacturing shared understanding.
If your game uses stats for monsters, avoid them, unless they manifest its oldness, sadness or hungriness directly within the rules. You will get by fine without your numbers; you have the truth of the thing.
An image will form. Do not dispel it.
Then, move on. They will never see this monster again.