The Collaborative International Dictionary of English v.0.48:

Zambo \Zam"bo\, n.; pl. Zambos. [See Sambo.]
   The child of a mulatto and a negro; also, the child of an
   Indian and a negro; colloquially or humorously, a negro; a
   sambo.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "zambo":
   Cape Colored, Eurasian, cattalo, citrange, cross, crossbreed,
   griffe, half blood, half-bred, half-breed, half-caste, high yellow,
   hinny, hybrid, ladino, liger, mestee, mestiza, mestizo, metis,
   metisse, mixblood, mixed-blood, mongrel, mulatto, mule, mustee,
   octoroon, plumcot, quadroon, quintroon, sambo, tangelo, tigon,
   zebrass, zebrule