Is it normal for a dog to do this?

While some 狗 are less 或者 更多 likely to do it than others, I believe this is a common habit amongst 狗 to hump their own 小狗 或者 other strange 'partners'-they have been known to do this with stuffed animals, siblings and even people...I am not 100% sure as to why they do this, but I believe it is a habit stemmed from curiosity, with some 狗 being 更多 prone to it than others depending on how randy they are 由 nature XD As a matter of interest, is Beast an older dog 由 any chance? I heard that senior 狗 are also 更多 likely to do it, though it is often simply a 显示 of dominance rather than anything else. Also if he is only 'mounting' and not anything...else, this is a common play gesture among some 狗 (although bearing in mind the pups are much smaller this is not such a good thing).

As for what 你 should do with him...well, neutering him will reduce the problem (although neutered 狗 may also perform this behaviour). Other than that, try removing him whenever he does it and say 'no' firmly, before distracting him with something else. In the wild, a female wild dog 或者 狼 will drive her partner away as soon as her pups are born, as unlike the mother the male dog rarely feels any attachment towards the pups when first born, and so are 更多 prone to unacceptable behaviour; perhaps 你 could keep Beast away from the 小狗 until they are bigger? 由 which time, he might perhaps have gotten used to the 小狗 and not be so inclined to...you know, and if they are bigger they may be able to stand up for themselves a bit more.

Actually, father/daughter 或者 mother/son matings often turn out fine. It really depends on the 狗 involved, if they are carriers for any genetic diseases 或者 not.
Inbreeding can be useful in breeding for specific traits, actually, so it's not always a bad thing. (However a breeder should have a pretty good understanding of genetics so that they know what they're doing before inbreeding.)