They have ears. They’re located just under their horns, obscured by their dark fur.
What they don’t have are pinnae—or none that we would recognize, anyway. Their horns essentially function as pinnae for houndoom, and, in fact, the going theory is that these were actually evolved from houndoom’s pinnae millions of years ago, likely as an adaptation in response to their pack structure. Oftentimes, houndoom determine pack leaders (as well as hunting parties) by engaging in battles with one another. The strongest houndoom—that is, the ones who are victorious in these battles—are the ones who hunt for food and lead packs to new breeding grounds. Thus, it’s likely ancient houndoom developed their horns, as those with horns have an easier time defending their positions within houndoom packs than those without.
The Pokédex According to Bill 