In the case of codominant inheritance, both alleles are expressed equally in the phenotype of the offspring. In your example, 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 offspring will have the genotype CWCB. Since the alleles are codominant, the resulting phenotype of the offspring will be roan, which displays both the white and black colors in their coat.
Therefore, the likelihood that their offspring will be roan, with both colors represented in their hair, is 100%.