I never knew what this was until I did some research on it, due to my cat's odd pupil shape.
Iris Coloboma is a defect of the iris, causing it to form improperly and giving the pupil a 'droopy' appearance.



Appearance
A diamond or keyhole shaped pupil instead of circular.
Pupil appears to 'drip' or 'bleed' down the iris.

Visible at birth?
Yes.

Effect gameplay?
Still up for debate. It is possible that this can cause poor eye sight later on in life.
(more research is needed)