I've started writing code while reading 5e, and looking through the data of the Bestiary. In the 3.5 days, a creature's HD was decided by its type -- Constructs had d10 HD, Elementals d8, etc. I started to note a trend in the latest playtest packet regarding this as well, but it wasn't tied so much to the creature type as it was the creature's size.
Generally, the rule seems to go like this:
- Tiny - d4
- Small - d6
- Medium - d8
- Large - d10
- Huge - d12
- (Gargantuan - d12)
This works for every creature in the latest playtest packet, except for 6/7:
- Demon - Balor
- Giant, Hill
- Rat, Dire
- (Roc, Giant)
The Giant Roc might not deserve to be in this list; it's the only Gargantuan creature in the Bestiary, and thus it's hard to know whether Gargantuans are also going to have d12 (we don't have a nice d14 or d16 to gently step up to, and a d20 HD? Ouch); that's why Gargantuan is parenthesized above.
So... am I right that there *is* a formula behind a creature's decision, and that these six are wrong? Perhaps they went through a size change during design, and their numbers forgot to follow along? Or perhaps the mapping of Size to HD is just a guideline, and not a hard rule. That would be a shame, though; A Balor has an average of 207hp right now, but by giving it the d12 it deserves, you can get near enough -- that exact average hp can't be the target at design time, can it?
The math on to-hits for monster attacks seems to have no rhyme or reason, but that'll be another post.