Doggy sounds childish. ‘How much is that doggie in the window?’ asks the popular song. (The song title used the spelling doggie, being American, though Britain enjoyed a cover version by Lita Roza in 1953, the same year as Patti Page’s original.). Doggo sounds cooler (like daddy-o in hep talk), but in the strange world of internet image-sharing it goes with a sentimentality which would shame the nursery.