Monsters with class levels

Strangely enough, this is all core. Here's the original:
http://paizo.com/pathfinderRPG/prd/monsters/monstersAsPCs.html
== How it works ==
Sometimes you may find yourself with a pixie whom you wish to add class levels. This is complicated, but remember
You brought this upon yourself
=== Step 1: Work out the creature's Equivalent Class Level (ECL) ===
This isn't hard. 90% of the time, it's the creature's CR. The exceptions are listed here:
http://paizo.com/pathfinderRPG/prd/monsters/monsterCohorts.html
Basically, certain monsters count as higher level. A pixie is CR 4, but we can see from the above table that a pixie has an ECL of 8.
Awaken increases a monster's CR by 1.
=== Step 2: Work out the creature's level progression ===
Hit dice become less important than class levels as the game ramps up. A Minotaur is CR 4, but it's obvious that while a Minotaur Fighter 1 is about on par with a Human Fighter 5, a Minotaur Fighter 10 is a lot less powerful than a Human Fighter 15.
How this is balanced is that monsters "skip" some levels. Every 3rd level, a creature gains an extra level, and does this a number of times equal to half its CR (rounded down). So, for our minotaur fighter:
# Minotaur Fighter 1. ECL 5
# Minotaur Fighter 2. ECL 6.
# Minotaur Fighter 4. ECL 7
# Minotaur Fighter 5. ECL 8
# Minotaur Fighter 6. ECL 9
# Minotaur Fighter 8. ECL 10
# Minotaur Fighter 9. ECL 7
# Minotaur Fighter 10. ECL 7
# Minotaur Fighter 11. ECL 7
Half our minotaur's CR is 2, so it gains a bonus level twice, on it's 3rd and 6th level. When it hits the 9th level, it gains levels normally.
=== Another example ===
Erika is a pixie. A pixie is CR 4, but we can see from the Monster Cohorts list, she's ECL 8. Half of her ECL is 4, so she gains 4 bonus levels, on her 3rd, 6th, 9th and 12th levels.