Our dog was one of a litter of ten puppies. About half of them looked like him: dark with brindle fur. The other half looked almost like golden retrievers: light fur with no brindle.
We asked our vet about this and she thought that there must have been different fathers of Dylan’s litter. It turns out because female dogs produce more than one egg it is possible (and not uncommon) for a single litter of puppies to have multiple sires if the female mates with more than one male in rapid succession while in heat (if you are to meet Dylan, please don’t mention this to him).
It also turns out that having multiple fathers per “litter” is possible anytime a female mammal produces more than one egg and has multiple partners during ovulation. Including humans. When a female human has twins with two different fathers, it is called heteropaternal superfecundation. How common is heteropaternal superfecundation? Three such cases were found in a parentage test database of 39,000 records. The frequency of heteropaternal superfecundation among dizygotic (fraternal) twins whose parents were involved in paternity suits was 2.4% in one recent study. Another study estimated that 1 in 400 fraternal twins are bi-paternal.