Not Ozarks specifically but I've been following big cat sightings across the UK for years and the pattern of reports you're describing sounds almost identical to what we get here in Shropshire and the Welsh borders. Large, black, low-slung, moving fast at dusk or dawn, usually near water or woodland edges.
The thing that always strikes me is how consistent witnesses are about the size. Not a domestic cat that looked big in poor light, not a fox. People who've seen these things are usually pretty clear that they know what a normal animal looks like and this wasn't it.
Do you have any details on the terrain around where it was spotted? And was this a single sighting or multiple people reporting around the same area over a short period? Clustering is really significant when you're trying to work out if you've got a resident animal rather than something just passing through.
