This thread lists all of the custom added familiars by the way as well as some very useful information about each familiar.
I never really understood why there are three different bats and only as familiars, two if you do not count skeletal bats.
I would not mind seeing a regular snake (like a viper or a large whip snake) as an animal companion and/ or a familiar.
The shadow asp is nice, but the fact they die almost instantly when exposed to daylight makes their use a bit restrictive.
At the moment the only time I still use animal companions or familiars is purely as a roleplaying prop though.
It just cost me too much to keep bringing them back to live. And a lot of them certainly do tend to die a lot.