housecat vs tortoise

housecat

noun
  • A pet cat that is not allowed outside 

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 housecat and tortoise occurred in a corpus of books? (source: Google Ngram Viewer )