There can be a tendency to normalize The Fantastic in RPG's. What should spark wonder, joy, or fear is instead treated as routine. One way this manifests is with creatures. There can be a tendency to simply state what the party sees, in mechanistic terms. "There's an ogre in the room." But in this description the wonder of what the thing is gets lost.
Describe instead what the characters see, or sense. It doesn’t have to be long. In fact it shouldn’t be long. But you should describe it. “You see an ogre.” is not a good description. “A grey hulking brute with massive muscling and long upturned from the jaw.” That’s a decent description. Put it in the read-aloud, or the DM info, or give the creature a one sentence description in your bestiary. You’re inspiring the DM and striking the fear of the unknown in to the players. There's some doubt. [In some ways this is related to 'show don't tell?]
Also, related to the principal of First things First, consider the monster entry in the bestiary, if you are including one. If the DM is looking up the monster it is most likely because it was just met. In these cases it makes sense to lead the creatures description with how it looks, smells, etc, and perhaps some notes on the attacks. It pays to be flexible and put things in an order they are likely to be needed in the adventure. What is almost NEVER appropriate is background and ecology information coming first. This sort of fluff/world building might be appropriate if the creature is to be reused, but the entry is not being used that way in the adventure; it's being used to run the encounter. And thus, the description is likely the thing that should come first.
