First thing to know is that Charizard’s egg group goes into Monster/Dragon eggs (which is strange as he is not a dragon type)

There is 79 pokemons being part of the monster egg group which is defined that their appearance is somehow beasty, reptilian or dinosaur.

The Dragon group egg consist of 52 which have the looks of a dragons, serpents; lizards, reptiles (somebody has to explain to me how Altaira fits in here someday).

What does this means, that Charizard to breed needs to be together with one pokemon that is part of the egg types, or with Ditto.