To write or say 'a elk' is simply grammatically incorrect when translating to English. In this case, it is simply the fact that elk begins with a vowel. Example: An elk; A moose. An animal; A plant.
"A", An", "One" can all be used in the singular, but if you're not a native or very fluent speaker of English, the proper use of articles can be tricky.
The use of the numerals is somewhat subjective, but in most cases typing "one" is preferred.