In the Spell Compendium, there is a spell Gutsnake that creates a tentacle coming out of the caster’s stomach. Then it states:
The tentacle created by the gutsnake spell is equivalent to a giant constrictor snake (MM 280) except that it is completely obedient to you and moves as you command.
Can this tentacle then receive the benefit of Animal Growth? On the one hand, it’s a tentacle. On the other, if it is treated as a constrictor snake except for specific limitations, then I don’t see why “Animal Growth” would not work.