So, we have horns on lions, wings and even golden claws, so how about a third eye?
My following ideas are:

.Just plain and simple, a third eye:


.Perhaps that's too unrealistic? How about having it designed like jewellery, on a golden / silver chain? Use your imagination to get more creative with this, my drawing is not the best:

.Different third eye colours! Just because your lion's eyes are green, doesn't mean the third can't be yellow ;)

Feedback / better demonstrations would be appreciated!
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Black jewellery idea: https://www.lioden.com/topic.php?id=304429372689