This is a red deer stag.
Having only visited Richmond Park once in my life, despite only being an hour's drive away, and not seen the deer there I thought it was time to remedy that situation this morning. The rutting season has passed, so I gather the stags should be a bit more chilled, and the hinds won't yet have youngster to protect. They are wild deer, but as the park is well used they are familiar with humans walking in the area.
It took a bit of walking but I found a gathering of a decent number of deer and very cautiously approached. I wanted to try get some photos of stag with a decent set of antlers on him. I watched one of them breaking the ends of some low branches on a tree with his antlers and manoeuvred by a fallen tree to watch. The stag seemed pretty relaxed and started grazing, even coming a little in my direction. I found I could walk slowly around without him bothering to lift his head, so he clearly wasn't that fussed by me being near.
I didn't want to push my luck and either scare him off or annoy him into getting aggressive (those antlers can do a bit of damage), but I still got fairly close. This shot was at 24mm on DX (crop sensor) and I've only very slightly cropped the original to give you an idea of how close I was.