Cavalier King Charles Spaniel vs Yorkshire Terrier

Heart disease isn't a risk for Cavaliers so much as a near-certainty — about half develop a heart murmur from mitral valve disease by age 5, and veterinary sources note that essentially every Cavalier that lives long enough will eventually develop it, making it the breed's leading cause of death.

Their tiny size is itself a health liability, not just a look — Yorkies are prone to airway collapse, liver shunts and severe dental disease largely because they've been bred so small, and 'hypoallergenic' is marketing rather than biology: no dog breed, Yorkies included, is truly allergen-free.