Sea Gambit is a Unicorn Pony.

She earned her cutie mark on a dark foggy night with a special spell.

Her guiding spell creates a thin string of light that finds other ponies so that they can follow it to safety.

The light turns white if it finds other ponies,
It turns yellow if it finds buildings.

She lives in a really foggy area to guide travellers through it.
She goes to the closest town for supplies, depending on which side of the fog she's on.

The basis of her cutie mark is "I'll leave a candle burning to guide you home"