I actually believe the ducky is the baby potty chair. If you catch your tama before it actually does the poo, then you get the potty chair. Later stages...teen and up actully get more of a regular looking toilet. I rarely catch my tamagotchis before they go poo, but when I do that is what happens.
You see wavy lines....you press the toilet icon, and the ducky potty(for babies/children) or reg toilet(for teens/adults) comes up....