This is a list of parts of Pokemon that are not connected, but may be able to potentially come off, even if it isn't show in it's sprite.
Parts that are not connected:
The things near the bottom of Cryogonal
Probopass's mini noses
Hoopa Unbound's arms
The back of Rotom Mow form (Note: I think it is not connected, however it is hard to tell.)
Parts of Gastly's body (Because Gastly is made of gas, all of that not connected purple stuff is still Gastly)
The rocks around Mega Steelix
Duskull's eye orb
Parts that may come off, but aren't shown to:
All of the Pokemon shown here.
The seeds on Sceptile's back.
That's all I could find.
Hope this helps!
Source1 (I just scrolled through the list.)