I got to this one in the second animals lesson, having only seen it as singular, and was marked wrong for giving singular 'älgen' w/no way to know or expect the plural was required. I don't want to report it if this question usually shows up in the lesson on plurals, but a (plural) note would be awfully helpful!
It's an annoying byproduct of how Duolingo works - it's a correct answer, so we have to accept it, but the system then decides arbitrarily to show it to you even though it's not the default answer - and even though you haven't seen the plural before. Unfortunately, we can't do much about that. :(