It's not really a surprise to find that Mieville was a D&D player in his youth; I bet the majority of fantasy writers under the age of 45 were. Slightly more surprising is that he's perfectly willing to admit to it in interviews, even if he always prefaces such comments by stressing that he has "no desire to start playing again now":
Probably one of the most enduring influences on me was a childhood playing RPGs: Dungeons and Dragons [D&D] and others. I’ve not played for sixteen years and have absolutely no intention of starting again, but I still buy and read the manuals occasionally....I still love all that—I collect fantastic bestiaries, and one of the main spurs to write a secondary-world fantasy was to invent a bunch of monsters, half of which I’m sure I’ll never be able to fit into any books.
It's refreshing to hear a fantasy author at least go some of the way to half-way thinking about possibly implying that playing D&D doesn't have to involve being a socially inept, geeky, aberrant freak.
At the same time, I think Mieville has completely the wrong end of the stick when he goes on to say that D&D is a creatively stifling influence on fantasy (something M. John Harrison also hints at; see below). From the same interview as the quote above:
[One of the other influences on me] of RPGs was the weird fetish for systematization, the way everything is reduced to “game stats.” If you take something like Cthulhu in Lovecraft, for example, it is completely incomprehensible and beyond all human categorization. But in the game Call of Cthulhu, you see Cthulhu’s “strength,” “dexterity,” and so on, carefully expressed numerically. There’s something superheroically banalifying about that approach to the fantastic.
I think this is a fundamental error, given the lie by the sheer volume of weird and wonderful stuff that RPG authors, designers, and more importantly DMs and players have come up with over the few decades during which the hobby has been popular. Trawl through the forums at rpg.net, dragonsfoot.org, or the many gaming blogs out there on the web, and you'll soon realise that the exact opposite to Mieville's argument is the case: the systematization that comes hand-in-hand with RPGs and monsters is a spur to creativity, not a hindrance.
In view of that, I'm going to go ahead and stat up a monster vaguely inspired by a creature from Mieville's book Iron Council:
The Moon Golem is crafted by only the most powerful archmages from the clearest light of a full moon. A silvery, intangible thing, with an outline somewhat like a man and somewhat like a bull, it attacks with horrible and inevitable slowness and grace.
HD: 14 (70hp)
No. of Attacks: 2
Special Attacks: If the Moon Golem successfully hits with both attacks it grabs its target and crushes for an additional 2d10 damage.
Special Defenses: A Moon Golen can be hit only by Cold Iron weapons, or weapons enchanted to +1 or more
Size: Large (8-10')
Morale: Fearless (20)
Other: The Moon Golem is non-corporeal, and can pass through doors and walls without impediment. It attacks the same target relentlessly and will not stop until it is destroyed or its target is dead (whereupon it will immediately focus on another). When it is a full moon the Moon Golem is at its strongest, and on such nights it gains 2 HD and increases its movement to 12.