If I'm not mistaken it's implied that Luca had something to do with it, but nothing indicates that he did it himself; he could have just got some cronies that were completely reliable (or, more likely, just scared shitless of what Luca would do to them if they told anyone). I don't have the novel on hand to check but IIRC it's something just like the Don asked that Luca Brasi see him, and that Hagen was confident that afterwards Woltz would inform them that Johnny had the movie part. The only part that could possibly indicate Luca did it himself is the note that whoever hacked off Khartoum's head must have been very strong, and perhaps the fact that it was hacked off with an axe (since in the flashback portion of the novel, Luca shows himself to be very handy with an axe as one unfortunate Capone henchman could attest, and the novel IIRC mentions that Luca was pretty strong)

While it's not canon by any means, the video game has your character (Aldo) and Luca go to the mansion; Luca goes into the stable and kills the horse and then you deliver it to Woltz's bedroom. Again though that's obviously not canon.