What is the lowest HD creature outsider with at will invisibility?

Question

A wizard using planar binder. Malconvoker so prefers evil outsiders. What's the lowest HD you need to get at will invisibility?

Answer 1

This answer covers Monster Manuals I through V and the Fiend Folio. I didn't hunt through additional material.

HD 2

The filth imp (Fiend Folio 100) has, among others, the following spell-like ability: at will—invisibility (self only). Because of their Hit Dice, three such creatures can be bound using the 5th-level Sor/Wiz spell lesser planar binding [conj] (PH 260-1).

HD 3

The imp (Monster Manual 56) and the quasit (MM 46) have, among others, the following spell-like ability: at will—invisibility (self only). Because of their Hit Dice, two such creatures can be bound using the spell lesser planar binding. Useful if confined to printed creatures in the core rules exclusively.

HD 5

The maurezhi (FF 50–1)—a ghoul-like demon—has, among others that are even more impressive, the following spell-like ability: at will—invisibility. It's also within the HD limit of the spell lesser planar binding.

HD 6

The fiendish (MM 107–8) ogre mage (MM 200) has, among others, the following spell-like ability: at will—invisibility. It also can be bound using the spell lesser planar binding. The application of templates is typically the domain of the DM, but this creature has the advantage of being possible according to the core rules.

HD 9

The couatl (MM 37–8) has, among a variety of others, the following spell-like ability: at will—invisibility. Its Hit Dice means that it can be bound using the 6th-level Sor/Wiz spell planar binding [conj] (PH 261) with 3 Hit Dice leftover for a also binding an imp or quasit (if the DM allows a different creatures to be called and bound with a single casting). The pair probably won't get along. Again, useful if confined to printed creatures in the core rules exclusively.

HD 15

A xerfilstyx (FF 59–60)—an aquatic, tauric, winged slug-devil—has, among a host of others, the following spell-like ability: at will—greater invisibility. Its Hit Dice means that it can be bound using the 8th-level Sor/Wiz spell greater planar binding [conj] (PH 261) with 3 Hit Dice leftover for also binding an imp or quasit (if the DM allows a different creatures to be called and bound with a single casting). It will get along fine with the imp but not the quasit, which it will probably torture then eat at the earliest opportunity.

Answer 2

The pixie (1RHD) has greater invisibility as a special quality, but is always NG.