Pugs have significant trouble breathing and cooling themselves due to their pushed-in facial structure.
Like Pugs, Bulldogs struggle to pant sufficiently making them very susceptible to heatstroke.
Brachycephalic Boxers don't cool efficiently through panting just like other short-nosed breeds.
Tiny Chihuahuas have a much greater surface area relative to their body mass causing faster heat gain.
Obese dogs have a harder time regulating their body temperature.
Puppies and seniors can't regulate their temperature well.
Very Young or Very Old Dogs
Heavy, dense coats trap heat unless brushed frequently.