I don't know, there is speculation that the Duck Billed
Playtypus is the result of a bird mammal hybridization, based on a close look at its genome. At least pigs and monkeys are both mammals, so if there could be a Playtypus from birds and mammals, a mammal to mammal hybrid must be at least as likely. The other thing to consider is the time scale for this to have occurred. Let's say that in an area with many pigs an monkeys, maybe once in 300 years a wild pig mated with a female monkey in heat. Let's then say that one in a thousand parings produced a viable offspring, if my math is right, you're only talking about 300K years. On the other hand, you have millions of years available for this to happen. Given those chances, lots of strange things could happen. It seems so to me anyway.