The first is dogs.
I'd pay to see you try to cause a successful union between a Great Dane and a teacup poodle.
The second is Man.
Man is amongst the slowest breeders in the world, and there has been no significant biological isolation of note for humans on this planet, by comparison to our leasurely breeding speed.
the third is horses.
Do you know what jennies and mules are? I suggest you look it up if you don't.
Yes I do know that they are cross species reproduction and I also know that the mules and jennies are sterile. They therefore have no bearing on the discussion.
As for the dog breeding you suggest I have heard of successes of such pairing. (With the male being the smaller dog)
It is estimated that man was on the American contents 10,000 years (I read an article recently that upped that to 16,000 years) before the Europeans arrived with a conservative generation of 20 years that is 500 generations. That is not huge on the evolutionary time table but not insignificant. Name a significant trait that natural selection produced in humans on the North or South American Continent.