So long as you are able to acqure a new familiar, you may choose you new familiar from a non-standard list. (see Familiars)
Prerequisite: Ability to acquire a new familiar, compatible alignment.
Benefit: When choosing a familiar, non-standard familiars are also available to the spellcaster. The spellcaster may choose a familiar with an alignment up to one step away on each of the alignment axes (lawful through chaotic, good through evil). For example, a chaotic good spellcaster could acquire a neutral familiar.
The improved familiar is magicall linksed to its master just like a normal familiar. The improved familiar uses the basic statistics for a creature of its kind, with these exceptions:
Hit Points: One-half master's total or the familiar's normal total, whichever is higher.
Attacks: Use the master's base attack bonus or the familiar's, whichever is higher. Use the familiar's Dexterity or Strength modifier, whichever is greater, to get the familiar's attack bonus with unarmed attacks. Damage equals that of a normal creature of that kind.
Special Attacks: The familiar has the special attacks of its kind.
Special Qualities: The familiar has all the special qualities of its kind.
Saving Throws: The familiar uses its master's save bonuses if they are better than the familiar's.
Skills: Use the normal skills for a creature of its kind.
Familiar Special Abilities: see Familiars.