As it turns out:
In a deleted scene just before Vito, Tom and Sonny discuss the upcoming Solozzo meeting, Vito asks Tom about his meeting with Woltz. After Tom tells Vito that Woltz is "not a Sicilian," Vito says, "Send Luca Brazi to me. I think we're going to find a way to reason with this Mr. Jack Woltz."
It's true that the novel doesn't identify Luca with the horse-killing. And, the deleted scene doesn't prove that Luca actually killed the horse--perhaps he organized it, but didn't wield the fatal axe. But, I can't blame Harlan Lebo for concluding that Luca did it.
This is one of those unfortunate mistakes FFC may have made. Luca was really no good at organizing anything. He couldn't even bluff Tattaglia Junior when he tried. Seems to me this was a job for Tessio or Clemenza. As for Luca...he could have ripped the horse's head off!
