Why did Papa Doc order the killing of all black canines in his country?

Clement Barbot started on a plot to remove Duvalier from office by kidnapping his children. The plot did not succeed, and Duvalier subsequently ordered a massive search for Barbot and his fellow conspirators. During the search, Duvalier received information that Barbot had transformed himself into a black dog. Duvalier then ordered that all black dogs in Haiti be put to death.

https://en.wikipedia.org/wiki/Fran%C3%A7ois_Duvalier

Indeed.