I dislike it when a DM disguises a monster without giving the players a chance to discover its true identity. A DM can withhold as much information as he wants, and as a result the players can be kept really clueless. This will only lead to two possible outcomes:
- The monster always gets a surprise round in combat
- Player characters trust nothing, and will just start murderhoboing everything, and stab strangers at the first opportunity
I find neither very appealing for my game.
If I want to trick my players, I make sure that they first commit a little sin. For example, there appears to be some gold on the floor. If greed takes over and they pick it up (usually in a rush before another player does it), then they get ambushed. I still get a surprise round for my NPCs, but now the players are bickering among themselves instead of with me. A much more appealing outcome.