For some modern people, as much as 5% of their DNA comes from neanderthals. Pretty-close species can have hybrid children, but the more distant the less likely the child will be fully-functional.
Consider mules (horse + donkey) which are sterile or the infamous Ligers and Tigons (Lion + Tiger can make some monstrously large cats).
I remember reading that Australian aboriginals have up to 5% of Denisovan genes (don't remember source, but it was widely reported after the discovery)
My question was how they could mate without having any morphological similarities. Horses and donkeys surely do!