Here was a boy [Keats], born the son of a London ostler, hungry for depth, for a kind of surging reality, for largeness and otherness which only epic poetry could provide.Why Homer Matters by Adam Nicolson
What is this strange ostler? A phonetic spelling of hostler that has been hanging around in English for over 500 years. Funny thing about English, some words come and go, while other just hang around, like zombies, refusing to die.
Hostler gives us hotel, youth hostel and hospital ... all very useful additions to the lexicon, but why has ostler remained after all these centuries. How did this happen? The answer can be guessed from Google.
Ostler has over 750K hits, while the seemingly more common, transparent and comprehensible hostler is under 500K. The answer seems to be that Ostler is also a common surname, while Hostler is not.