big cat vs tortoise

big cat

noun
  • Used other than figuratively or idiomatically: see big, cat. 

  • Any large feline animal, especially one of the Panthera genus, including tigers, lions, jaguars and leopards. 

tortoise

noun
  • Synonym of cat (sense 10, a wheeled shelter) 

  • Any of various land-dwelling reptiles, of the family Testudinidae (chiefly Canada, US) or the order Testudines (chiefly UK, Australia, New Zealand, Ireland, South Africa, India), whose body is enclosed in a shell (carapace plus plastron). The animal can withdraw its head and four legs partially into the shell, providing some protection from predators. 

How often have the words big cat and tortoise occurred in a corpus of books? (source: Google Ngram Viewer )