i've often been able to successfully explain Sonas, fur or otherwise, to people outside of the (generally queer/furry) culture with:
The (mental) image of yourself that makes your heart feel Smile the most.
if you like being represented by yourself! tadah! that's your sona! it's you!
if you wanna be a large sparkly space robot/AI who protects newly-formed planets from space debris, tadah! that's your sona! that's you!
it's a way of putting it in a perspective of self-enjoyment/fulfillment/love for people who Just Don't Get It on first contact.