Thanks to everyone who identified the exotic bird at Rushy Common. The consensus is that she was a female melanistic common pheasant, in particular Lindsay Fisher writes:
Fascinated by the Exotic Bird post. A couple of years ago, possibly BP (Before the Pandemic), I spotted what I was convinced was a jet black pheasant in the Windrush watermeadows just beyond our garden (this is the Grimesmead nature reserve, managed by the Wychwood Project). It was a momentary sighting, and I wasn’t able to photograph it, but the shape of the tail feathers was unmistakable. I looked out for it in vain for weeks afterwards, but our neighbour confirmed I hadn’t been dreaming when he said he too had seen it in his garden (sadly, again undocumented). He said it seemed almost an iridescent blue-black at close quarters. After a bit of research, I learned that these melanistic variants are not that unusual in pheasants. Certainly startling, though. I wonder if the Rushy Common sighting is any relation of the one I saw?
Lindsay Fisher, March 12 2022