Kobolds and Goblins have no LA and only 1 HD. When they take a class,
their class level replaces their HD.
Catfolk have a +1 LA and only 1 HD. Like Kobolds, Goblins, and the PHB races, their class determines their HD, but there's a catch: they start a level lower. In the case of a 2nd-level game, the PHB races will all be level 2, but the Catfolk will be level 1 because its racial abilities are more powerful.
Gnolls have a +1 LA and 2 racial HD, so a plain gnoll is treated as a 3rd-level character. If a creature has more than 1 racial HD, you can't replace those HD with class levels, which means you're stuck with them. Racial HD beyond the first are
generally a burden, in that they're inferior to class levels. Case in point: gnolls are CR 1 but they're treated as 3rd-level characters.
Edit: You could work out a progression for the Catfolk and Gnolls. Savage Species has a lot of examples (including a gnoll, I think); they essentially take "levels" in their "racial class", and have to finish it all the way until they're a full-grown version of the creature. Here's a sample progression, which I whipped up on the spot:
| A 1st-level kobold sorceror, for instance, has a single d4 hit die, just like an elf, human, or halfling sorceror of the same level would. |
Catfolk have a +1 LA and only 1 HD. Like Kobolds, Goblins, and the PHB races, their class determines their HD, but there's a catch: they start a level lower. In the case of a 2nd-level game, the PHB races will all be level 2, but the Catfolk will be level 1 because its racial abilities are more powerful.
Gnolls have a +1 LA and 2 racial HD, so a plain gnoll is treated as a 3rd-level character. If a creature has more than 1 racial HD, you can't replace those HD with class levels, which means you're stuck with them. Racial HD beyond the first are
| Most racial HD are like taking levels in the Warrior NPC class, which sucks. Dragons and Outsiders are the only things with racial HD that don't completely suck. |
Edit: You could work out a progression for the Catfolk and Gnolls. Savage Species has a lot of examples (including a gnoll, I think); they essentially take "levels" in their "racial class", and have to finish it all the way until they're a full-grown version of the creature. Here's a sample progression, which I whipped up on the spot:




