Would this be more fitting in the context of "there are not even one hundred horses here" or "not even one hundred horses could pull me away", or does it fit both?
Ens actually just means "even." Entirely separate word. From what I can tell, the article is often just understood when talking about numbers. If I'm correct, the only time you would want to use a definite article with a noun described by a number is when it's at the very beginning. Otherwise, you would say, "Jag har hundra hästar." Do keep in mind that when English speakers say, "a hundred," we really just mean "one hundred," just like we would say, "two hundred." Same with Swedes.
What the heck does ens mean, did I miss something? I do not recognize that word at all
Swedish often says just "hundra" for "one hundred". So here the "ens" does NOT mean "one". (Because "hundra" is a neuter word, it would have to be "ett hundra" if the word "one" were used at all.)
Here the word "ens" means "even". So, for example:
ens två hundra hästar = even two hundred horses
An alternate translation -- perhaps a better one -- of the DL Swedish sentence here is: "Not even a hundred horses".