Partial HD advancement

[Bad Wolf]

Say you have an animal companion with half a hit die. How does that work with the general progression? Do they have 2 and a half hd at third level?

[Kayblis]

One "level" is a HD. Creatures with less than 1HD that advance in any way go to 1HD, as if it didn't had any Hit Dice. This is like a humanoid getting his first class level, as a classless humanoid like a Human or a Dwarf has 1 "Humanoid" HD which he loses as he gets his first class level.

[theblasblas]

To clarify, does that mean that upon reaching lvl 3, a Druid with a cat as an animal companion will have a 2 HD cat?

[BowStreetRunner]

One "level" is a HD. Creatures with less than 1HD that advance in any way go to 1HD, as if it didn't had any Hit Dice. This is like a humanoid getting his first class level, as a classless humanoid like a Human or a Dwarf has 1 "Humanoid" HD which he loses as he gets his first class level.
I've located the rules where it says creatures with 1 HD or less adding a class level (http://www.d20srd.org/srd/improvingMonsters.htm#monstersAndClassLevels) do this, but nothing about when they gain HD in any other way.

[Necroticplague]

Say you have an animal companion with half a hit die. How does that work with the general progression? Do they have 2 and a half hd at third level?

Half a hit die is one hit die, just with its result divided. So gaining two HD would put you at three HD.

[BowStreetRunner]

Half a hit die is one hit die, just with its result divided. So gaining two HD would put you at three HD.
I'd love to see actual rules on this, but I don't think the designers were explicit in explaining any of this.

So do you get to reroll your first hit die? Creatures with 1/4 HD or 1/2 HD aren't going to be as tough if they are stuck with a 1/4 or 1/2 HD worth of hp on their first HD.