I feel the best way to explain what's happening is to just describe the process of doing it, with plenty of pictures.
-Open the Petz Creator
-Under the dog tab, select "Blank"
-Swap the mouth to this wide Boxer/Bulldog styled one
-Paintcan fill the pet with 1 color
-Toggle shadows off and back on
-Swap the mouth back to the original
-Paintcan fill the pet with a different color
-Toggle shadows off and back on
-Hit the undo button until...
-??? nose: reduced to 2 pixels and a shadow.
-This pet can now be colored, have other body parts swapped and modified, shadows turned on/off, etc normally, with a weird nose.
-In theory, this can be done with any pair of body part variants. 2 noses, 2 heads, etc. Some probably have no change, and some are probably more "useful" than others, but I haven't rigorously tested this. Doing it with outlines on the edge of the pet causes parts of them to turn transparent.
Forgive me for rather large screenshots I hastily reduced the size by 50 and prayed it was good enough.
Edit: Paint can filling is, as suspected, not necessary. Toggling shadows off/on is what causes this.
-Open the Petz Creator
-Under the dog tab, select "Blank"
-Swap the mouth to this wide Boxer/Bulldog styled one
-Paintcan fill the pet with 1 color
-Toggle shadows off and back on
-Swap the mouth back to the original
-Paintcan fill the pet with a different color
-Toggle shadows off and back on
-Hit the undo button until...
-??? nose: reduced to 2 pixels and a shadow.
-This pet can now be colored, have other body parts swapped and modified, shadows turned on/off, etc normally, with a weird nose.
-In theory, this can be done with any pair of body part variants. 2 noses, 2 heads, etc. Some probably have no change, and some are probably more "useful" than others, but I haven't rigorously tested this. Doing it with outlines on the edge of the pet causes parts of them to turn transparent.
Forgive me for rather large screenshots I hastily reduced the size by 50 and prayed it was good enough.
Edit: Paint can filling is, as suspected, not necessary. Toggling shadows off/on is what causes this.