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:
Haunter's Hands
Exeggcute
The things near the bottom of Cryogonal
Probopass's mini noses
Shedinja's halo
Klinklang's rings
Porygon-Z's head
Claydol's arms
Hoopa Unbound's arms
Hoopa'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.
Magneton
Dugtrio
Tropius's bananas
That's all I could find.
Hope this helps!
Source1 (I just scrolled through the list.)
Source2
Source3