Greek Kynósoura was used for Ursa Minor. Its Greek elements read as dog and tail. English cynosure developed senses for a guide and a focus of attention alongside its celestial uses.
The reason for the constellation name is unknown. Merriam-Webster allows possible reshaping of an unidentified borrowed name. English borrowed through Middle French and Latin; the display selects only some stages.
Sources
- Merriam-Webster: cynosure
Word History gives the Greek elements and explicitly qualifies the name's deeper origin; Did you know? explains the later figurative senses.
- Online Etymology Dictionary: cynic
Also built on Greek *kynos* (genitive of *kyōn*) "dog": cynic
- logeion.uchicago.edu · κυνόσουρα
Meanings of the older forms on the word path.