Trick Monster: This applies to any variation of a listed monster, such as: A two-headed giant ogre, a carrion crawler that walks upright and has paralyzing tentacles for arms, a wild bore (a shaggy man who tells long, dreary stories), a quartering (half-sized halfling), a Mouth Harpy (who can't sing but plays the harmonica), an Ogre Jelly (looks like an ogre, but . . .), and so forth.
I came across this last night while stocking one of the levels of Sangmenzhang, and it raised a wry smile, but I have to say my kind of D&D doesn't really tolerate this kind of thing. I get the joke, I get that not everything has to be po-faced in any RPG, but still, it's an incredibly unsubtle way of crowbarring humour into a game.
I have two objections. The first is the simple one: every single RPG session I've ever been involved in, as player or GM, has more laughter in it than serious moments at a ration approaching 2:1. This is achieved without anything substantively "funny" in the rules, set-up, or content of the game. It's just because that's the way gaming is - funny things happen, and when they don't, funny things can be said about the things that are happening. So actually, I've never felt any need to insert pre-planned jokes or make any self-conscious attempt at humour in the setup of a game or session. It happens organically anyway.
The second is more philosophical, and is that jokes and puns are a kind of Brechtian alienating device which distance the players from the game world. As soon as a Mouth Harpy makes an appearance, suddenly you're not even remotely inside your character's head - your willing suspension of disbelief has been shattered and you've been wrenched out of the Secondary World which has been created. This is not really conducive to what I enjoy about gaming.
What this boils down to, I think, is that good humour is generally about what the Japanese call the tsukkomi and the boke, or what we call the straight-man and comic: you need a sensible springboard for the jokes to bounce off. For me, whatever the game is you're playing, the game's contents have to take on the role of straight-man - they have to be relatively serious on the face of it for the jokes to riff on. Seriousness of content is the necessary foil for all manner of jokes, sarcasm and piss-taking - and the overall level of humour in fact suffers without it. That is, overt humour built-in to the game world undermines it: to borrow again from theatrical comedy, there is no-one "feeding" the lines, because the lines are all stupid jokes in the first place.