In codominant inheritance, both alleles are expressed in the phenotype of the heterozygous offspring. In this case, the homozygous white horse has the genotype CWCW and the homozygous black horse has the genotype CBCB. When these two horses are bred together, all of their offspring will have the genotype CWC. Since both alleles (CW for white and CB for black) contribute to the phenotype, the offspring will exhibit a roan coloration, which displays both white and black hair.
Thus, the likelihood that their offspring will be roan with both colors represented in their hair is:
100%