An upcoming attachment editor for Creatures3 and DockingStation for Linux and Windows.
The .att files describe the points where the different bodyparts are attached, e.g. where the head sits on the neck.
To change the attachments you open up a sprite file and an existing attfile. The points are marked with a cross. You can drag those crossmarks around and of course can save the attfile when finished. You don't even need to know those funny letter and figure codes used by sprites and attachment files for creatures. It's easily done by dropdown boxes, where you can select the bodypart, lifestage, gender/breed and slot/race.
Though you only need to create your own set of attachment files if your new breed differs of the standard breeds. Moreover the official breeds come only with a set for the males since the females use those files too if the engine doesn't find their set.