Improved Familiar [General]
This feat allows spellcasters to acquire a new familiar from a nonstandard list, but only when they could normally acquire a new familiar.
Prerequisites: Ability to acquire a new familiar, compatible alignment, sufficiently high level (see below).
Benefit: When choosing a familiar, the creatures listed below 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).
Improved familiars otherwise use the rules for regular familiars, with two exceptions: If the creature's type is something other than animal, its type does not change; and improved familiars do not gain the ability to speak with other creatures of their kind (although many of them already have the ability to communicate).
The list in the table above presents only a few possible improved familiars. Almost any creature of the same general size and power as those on the list makes a suitable familiar. Nor is the master's alignment the only possible categorization. For instance, improved familiars could be assigned by the master's creature type or subtype, as shown below.
Source: SRD 3.5 Feats