Well, I could be wrong too... but I think they sound like hot spots from licking... either a "moist dermatitus" or if worse, a "lick granuloma"... maybe... they are somewhat common in dogs... and if they are located near their butt, they are often from flea bites.
My nervous dog Gidget gives them to herself when she is left alone too much... but those obsessive-compulsive-nervous type hot spots are more commonly on their front legs, an easier place to reach. There isn't anything causing her itching, it is just a nervous habit, like nail biting.
Hot spots on the butt are just usually caused by an actual bite there. And cats rarely develop hot spots except from a flea or other allergy. Left untreated, my Kitty number 6 would lick himself bald. If there is a flea or other allergy, the cat's lymph nodes on his belly near his BACK armpits will be swollen and you can feel them, like golf balls under the skin.
Part the hair and scratch hard down to the skin right down their spine, particularly near their rump. if you see any flea "dirt" (black stuff is actually flea poop) then that would be my best guess, even if you don't see actual live fleas.
http://www.thepetcenter.com/gen/itch.html