Add-onThere is an add-on for the game, Impossible Creatures: Insect Invasion, available from Relic's official IC site for free (size: 88 Mb).
EngineThe game's graphical rendering engine is called SPOOGE. There is a separated physics/collision, AI/script, and combiner engine too.
TitleDuring development, this game actually underwent no fewer than two official name changes, first from just plain Sigma to Sigma: The Adventures of Rex Chance and then finally to Impossible Creatures as we know it now.
Information also contributed by Rev-san