Yeah.
I reckon Ditto's Mew's closest ancestor, because it probably has the DNA of all pokemon, as it can use all moves (by Transform) and breeds with everything that can breed.
I hope in the "National" order of whatever country Isshu's in Mew and Arceus are closer together.
Really, the problem is that things aren't linked. Sure, Arceus created the dragon trio, but who created Groudon, Kyogre and Rayquaza?
I consider most "legends" as more of "deities". Lugia, Ho-oh, Celebii and the 3 beasts are legends.....
The dragons are just rare/deities.