edit: apparently it's Bill Baker of Microsoft who originated it: https://www.pistoncloud.com/2013/04/announcing-enterprise-op...

Afaik the analogy was first used by Joshua McKenty of Piston Cloud

'"The servers in today's data center are like puppies -- they've got names and when they get sick, everything grinds to a halt while you nurse them back to health," Joshua McKenty, co-founder of Piston Cloud, is quoted as saying in a recent company press release. "Piston Enterprise OpenStack is a system for managing your servers like cattle -- you number them, and when they get sick and you have to shoot them in the head, the herd can keep moving. It takes a family of three to care for a single puppy, but a few cowboys can drive tens of thousands of cows over great distances, all while drinking whiskey."'[1]

[1] https://www.networkworld.com/news/2013/041113-servers-cows-p...