A cat may look on a king.
The saying famously shows up also in Heywood's 16th-century proverbial poem (a treasure-trove for proverb hunters):
Some hear and see him whom he heareth nor seeth not
But fields have eyes and woods have ears, ye wot
And also on my maids he is ever tooting.
Can ye judge a man, (quoth I), by his looking?
What, a cat may look on a king, ye know!
My cat's leering look, (quoth she), at first show,
Showeth me that my cat goeth a caterwauling;
And specially by his manner of drawing
To Madge, my fair maid.
It also famously appears in Alice in Wonderland.
You can also find the proverb in English Proverbs and Proverbial Phrases by William Carew Hazlitt (Google Books). The poster is made with AutoMotivator. The image is of the Cheshire Cat by John Tenniel at Wikipedia.