Per Wiki Cougar (SLANG)

The origin of the word cougar as a slang term is debated, but it is thought to have originated in Western Canada and first appeared in print on the Canadian dating website Cougardate.com. It has also been stated to have "originated in Vancouver, British Columbia, as a put-down for older women who would go to bars and go home with whoever was left at the end of the night."

So, there you go. It is appears to be a Derogatory term.

IMO,
It originated from comparing a house cat playing with a ball of yarn to an older experience woman finding a young man to play with. Derogatory or not it is fun to say.

Cougar! Cougar! Cougar! Cougar! Cougar! Cougar! Cougar! Cougar! Cougar!

Because the boys are more tender and tasty than older humans, making it easier for the cougars to eat.