it's the circle of life my friend, as a new cubone is brought into this world, his mother dies, leaving him her skull and femur as memoriam, his birthright so that she would never be apart from her son she never got to meet but loves just as dearly so
Last one is digiegg of kindness, which I don't think is ever seen outside of the card game
, second last is the golden armour egg used for terriermon in the digimon movie.
I think the second last one had something to do with patamon or the little cat digimon whose name I can't remember. I have just started re-watching the show starting from the first episode.
Patamon and the little cat one digivolved into Angemon and Angewoman and were able to summon the golden eggs to give the other digimon golden armor. (Sorry for ****** ass grammar)