Mutations happen, and aren't all that rare- my favorite is Apache, the "macchiato" horse
, who seems to have a mutation that caused a coat pattern and
a dilution effect. Not only do his parents not have any dilution (they're rather plain looking bays) but there's no dilution present in his breed at all. Apache is gelded, but if he weren't he could theoretically pass on his mutation, which would be a new gene.
Cream probably happened a long time ago (and likely in more than one horse), but there are plenty of breeds in which cream is still not present.