Ah yes, it looks like I've used the wrong tag there and it should be something like landuse=meadow + meadow=pasture? That doesn't label the use for horses, though, but indeed an equestrian area is usually round or rectangular and not this odd shape. I don't think it would be too small (~20m diameter on the semi-round west side), just the shape is odd. Does pasture meadow seem correct to you then or should it be something else?