I know its a technicality, but an important one. While the mansion is the one used for the exterior shots, it was not the site of the famous horse-head scene. That site was on the other side of the country.

The bedroom of Woltz’s villa, where the studio boss finds the head of his unfortunate horse, is the living room of Falaise, on Sands Point Preserve, the Guggenheim estate way out at 95 Middleneck Road on Long Island