Descriptive and narratives drive the game and this is especially true when it comes to encountering a monster.
Speaking as the DM, I always avoid throwing out the Monster Manual name of the monster even if it is just an orc. Take the time to briefly describe what the players see and let them give a name to the monster.
This breathes fresh life into common monsters, and adds even more of an air of mystery to the uncommon ones- plus it keep that player who has the Monster Manal memorized still guessing.
I’ll modify my description based on the relation and situation to the party. If a group of orcs is far off in the distance I’ll describe that they are wearing darkened heavy armor and armed with shields and long pole arms.
If the party was up close I would add that the armor is rusted, I’ll fitting, and falling apart- perhaps suggesting a smaller armor bonus due to its disrepair.
Players also tend to fill in the encounter gaps with their knowledge, and we want to take advantage of that, by letting them take a guess at what they are encountering based on their real time past game experience.
See you in the game!