Not intentionally or publicly, sure. My line of thought is that it is possible that some "left hand not talking to the right hand" stuff went on. Dev team adds Bornean Elephant and Pere David's Deer to their internal list of semi-aquatic species. Unrelated to this, the website team gets instructed to prepare the next challenge page for semi-aquatic species, so they refer to the internal list without realizing that some of them may still be a work in progress, and bam. Not like the public is expected to notice this list, so perhaps there's a minimum amount of oversight and double checking. And then, of course, as long as the DLC doesn't release before the challenge, those species will naturally not be referenced in the in-game challenge.Honestly: I doubt Frontier would include animals in a community challenge that aren't released yet.
I'm not saying that this is what happened. I'm just saying, it's probably not impossible.