@ShadowLightMew It's one code per game, unfortunately. At least you can trade the Volcanion itself over and back for the 'dex entry.
Meganium certainly takes more design liberties than fossil Pokemon, but its stance has always reminded me of a Brachiosaur with a short tail:
I am totally biased, though.
(It seems to me that having no tail to counterbalance the neck would create balance issues, but who cares? This is Pokemon, not Palaeo class)
Tropius has the same sloping back and stubby tail, but it's armour plates remind me of
Titanosaurs, a sister clade to Brachiosaurs.