Before I go on, I should add that you're quite right about nasnas getting class levels... but I'm not sure about them gaining Hit Dice as the years go on.
Mostly because they are not expected to live very long. I suspect wizards and clerics would come up with a way to expend more resources to create them with more Hit Dice. Maybe spellcasters could enchant a fruit for consumption that would create a 2 HD version, a 4 HD version, and a 6 HD version.
Less important, I keep speculating that these things probably don't grow uber-rapidly after birth (such as infant to fighting-form in a matter of days), but perhaps still quite fast (perhaps over the course of one year).
...How about this for a beginning?
Medium Aberration
Hit Dice: 2d8+4 (13 hp)
Speed: 20 ft. (4 squares) [includes some sort of armour - studded leather?]
Armor Class: 14 (+1 natural, +3 studded leather), touch 10, flat-footed 14
Special Attacks: Fear aura, monodextrous
Special Qualities: Darkvision 60 ft., DR X/cold iron or magic
Abilities: Str 20, Dex 11, Con 15, Int 7, Wis 8, Cha 12
Feats: One plus Endurance (B)
Skills: Jump (with racial bonus) plus what else?
I'm not convinced yet that since they are fairly twisted and insane that they should have a Wisdom score with a negative modifier... in fact in this case they might be all the more strong willed because of their determination to simply remain upright and dangerous. That said, I wouldn't argue against a low Wisdom for long.
Regarding their ability to cause fear: Should we assume that under most combat situations, nasnas will hoot and bark and scream, therefore they have a fear aura... or should they have this as a Spell-Like Ability used at will or a number of times per day?
Also, should they get a bump to the Will save DC against the fear effect for multiple nasnas in the area who are screaming and hooting?
Regarding the Damage Reduction, will DR 5 be too lean... or is DR 10 getting too tough?
Regarding skills, I imagine they would have a racial bonus to Balance as well... but I might be too generous.
