So these are two Pokemon that have breeding oddities. Now I know the reason is because Gen 1 has no breeding mechanics, so a lot of how Pokemon reproduce was pretty much an unknown.
We see this in the anime as we have things like "Baby Pikachu" and "Baby Nidoqueen", and though they're blink and you miss it moments, they're still a solid case of Early Installment Weirdness.
But still, I'm a total dork, so I have to have my head cannons for things that don't make sense.
So if you don't know what's wrong with these two Pokemon when it comes to breeding, I'll make it simple. Cubone's a longer explanation so I'll start with it.
Cubone's most infamous trait is the fact that it is constantly depressed, wearing its mother's skull and pining for a parent. Eventually the skull on Cubone's head fuses with the creature's real skull, and Cubone masters using the bone as a weapon, giving us Marowak.
Marowak if female can lay a Cubone egg, the Marowak does not die, but the Cubone is STILL wearing the mother's skull... Okay weird, and even stranger, Cubone can have siblings ALL WEARING the skull of its mother, how the heck does that work?
And for Kangaskhan, the issue is easier to explain. Kangaskhan hatches from eggs as a full grown adult with a baby in her pouch. If that makes sense to you, then I need some of what you're on.
However I can resolve both plot holes in one fan theory, and here it is.
Kangaskhan and Baby Kangaskhan are not mother and child, they are SIBLINGS! The bigger one is always female, the smaller one, not so much. The two are born together and start off with a symbiotic relationship; the nature of which is commonly mistaken as being of a mother and child by those who don't know any better.
At some point, there was a gene that caused a birth defect in which the older sibling dies in utero or improperly forms, leaving behind mere bones. The younger one having the instinctual need to form this symbiotic relationship, wears the skull and bone to compensate.
At some point, Kangaskhans without this defect outbred the ones that had it, making it virtually extinct. Causing Kangaskhan and Marowak to divulge into two separate species. Cubones and Marowak that still have the defect, making it impossible for them birth a healthy Kangaskhan, and Kangaskhan gaining an immunity to the problem making it impossible for them to birth a Cubone.
I mean yeah there is the Cubone who misses his mother in the first game, but, we see the mother's ghost is a Marowak. So... yeah..
What do you think? Have you come up with a different theory, the same theory, or have you just not thought about it?
Lemme know
Francis wasn't evil