I'm quite over the moon with the series of racial classes that Necromancers of the Northwest have been doing lately. So far, they've released
the nymph,
the hound archon, and
the succubus as part of their "Races with Class" series of articles.
They go into their design philosophy regarding building these racial classes
in this article, but the bottom line of it is that they've adopted a particular stance in regards to these: they break the base monster down into however many levels encapsulate its stats, and then add as many levels as necessary to reach an even twenty.
The idea here is that, once the monster reaches enough levels to be equal to the base monster, it can then multiclass or continue its racial progression. If it chooses to continue, it gains new powers and abilities as an advanced member of its species. It's quite ingenious, and I really love seeing the new powers they've invented for the races they've done so far.