Ni's eye

Two days ago, I noticed that Ni's right eye's pupil was twice larger than the left. There was no swelling or discharge. Ni was in his normal, good behavior. After about 10 hours, the condition disappeared and has not recurred. Anyone has any idea of what could have caused it? I will bring him to the vet if it happens again. 

Did he recently hit his head or something? Sounds like it could possibly be a slight case of horner's syndrome... but if it has already disappeared it probably isn't since that usually lasts for a few weeks, but it is not a "dangerous" condition and usually resolves by itself without any sort of treatment... it just looks creepy. He may have just banged his head on an area near the brain that controls eye functions and stuff that caused that temporarily. If it happens again or his third eyelid starts popping up/eyelid starts to droop I would call the vet and let them know and see what they have to say/if they want to see him, but for now just keep an eye on him... I wouldn't worry about it for now if it is gone and he is otherwise acting normal.
 

Thank you very much! So far so good...