This is possibly the one discussion we haven't had on this forum.
I know pseudo dragons (LG,NG) have excellent magic resistance, and fairy dragons have invisibility (as well as excellent fashion sense...) and the cat (CN) can pickpocket for you, but what can the
ferret and rabbit do?
what can the imp, dust memphit and quazit do? I know you can abuse the quasit, but can it do a special ability?

The mephit can cast glitterdust. The ferret can pick pockets too. The rabbit has stealth.

I remember glancing through them using Infinity Explorer, but I didn't pay a whole lot of attention. My dust mephit spent most of his time in the Bard's backpack. I took him out, feeling sorry for him and he got himself killed. I didn't bother to reload. Oh well.

What familiar has the best stats for pickpocket ? I have the cat and never he succed it ! Unless it is fish!

Imps can cast Polymoth self,
quazit Horror and somthing else i think blur

The ferret is the better pickpocket'er. The cat is more of a scout than anything else.
The rabbit detects 50% of traps.

Oh yea, Fairy Dragons also do 1 point of electical damage per hit.

Ok I have a question, whenever I try to cast a familiar, it says I need to be a protagonist. I'm of true neutral alignment, and can never cast one. How do I become a protagonist?

Your protagonist is your PC, and he/she has to be a mage (single/dual/multi) in order for find familiar to work. Casting from NPCs will cause that message to appear.