Still thinking about domestication. Mostly, the wild ancestors of domesticate animals were social: presumably, such behavioral tendencies were preadaptations that helped the domesticates come to bond with or at least tolerate people. Social animals can have adaptive personality variation – difference behavioral strategies. Sometimes those strategies are facultative, sometimes genetic, sometimes a mix.
I would guess that those wild individuals that account for most of the ancestry of f a domesticated species didn’t have a representative mix of personality types. Probably they were more likely to be followers rather than leaders – not the alphas of the pack, not the most aggressive stallions. Sidekicks. With dogs, we can probably check this hypothesis fairly easily, since wolves are still around.