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.
As a player I've been tricked by a town of house hunters, house-sized 20 or 30 HD mimics from 2e. "Did the whole block just move a little closer to us?". Not very cute but I was tricked.
I've never seen players as terrified as my entire group was by a "completely innocent looking/harmless" eight year old girl loose in the lower levels of a dungeon.
OMG you killed Angela Lansbury!!!
That was an adventure in Dungeon magazine wasn't it?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.