Sorry for taking a while, I was busy with irl stuff.
In short, some mythicals originate from humans being modified to be more animal-like, and some were animals modified to be more human-like.
P and Argos, for example, both originate from lineages of humans who were transformed. As such they can still have kids if they pair up with humans, and sometimes the "mythical gene" can skip a generation.
Meanwhile Themba and the grootslangs do not descend from humans in any way, so they can't have kids with humans.
That's what I meant for the longer answer.