Wikipedia tells us that "the elk, or wapiti ... should not be confused with the larger moose (Alces alces) to which the name 'elk' applies in the British Isles and Eurasia."
I prefer the guidance given by Canadian Rangeland – a company that markets bison and elk meat: "the names given to North American animals by European settlers weren't always that accurate ... elk – the european word for moose – was used for wapiti which are actually a type of deer."
Canadian Rangeland also points out that "North American bison [commonly known as buffalo] are only scarcely related to true buffalo". *
Wikipedia's summary seems to suggest that the creature known in Eurasia as an elk should really be called a moose – which strikes me as somewhat perverse, as an elk was an elk long before any English speaker had even seen a moose.
What actually happened, it seems to me, was something like this: when the European Americans saw a wapiti, they thought it was an elk. Then they saw an elk, but they already had something called an elk. So they asked the Native Americans what they called the elk, and the Native Americans said they called it a moose. And so the elk came to be known in North American English as the moose.
Nothing wrong with that, really – until they start implying that it's the Europeans (and particularly the Brits) that have got it wrong.
* Coming, as I do, from the English West Midlands, I can't resist introducing at this point one of my favourite jokes:
"What's the difference between a buffalo and a bison?"
"Yer cor wash yer 'ands in a buffalo."
© Haydn Thompson 2017