So we start with the Gengar sprite in the original Red/Green/Blue games...
Just a normal fat ghost, though with white eyes. The Red/Green artwork mostly resembles the Gengar we know today:
...then when they redrew the Pokemon for Blue, they made Gengar, well, blue for some reason:
By the time the anime rolled around, Gengar was back to purple:
And for Yellow, Gengar's sprite was redone to bring it closer to the anime, like all the other sprites in that game:
Gengar remains pretty consistently purple, but of different shades. Compare their Gold/Silver sprites to their Crystal sprites, which is slightly darker:
Gold/Silver
Crystal
In gen 3 the purple becomes much more saturated:
Then, interestingly, gen 3 began a trend where Gengar's back spikes start getting drawn differently on back sprites. Only a small handful of Pokemon in gen 3 had different back sprites between Ruby/Sapphire/Emerald and FireRed/LeafGreen:
RSE
FRLG
The back sprite would continue to have disputes on how to draw the spikes into gen 4:
Diamond/Pearl
Platinum
Again, very few Pokemon had redrawn back sprites between Diamond/Pearl and Platinum, so clearly this was notable enough for the sprite artists to take a stand on. Ever since Platinum, Gengar's back spikes have pretty consistently take on the "mesh" look and lost the pointed needles from previous sprites (though they're much less pronounced in gen 6):
Gen 5
Gen 6
...but I must again point out that they changed Gengar's color again in gen 4. Compare the shade of purple used in DPP to HGSS:
Diamond/Pearl
Platinum
HeartGold/SoulSilver
They also recolored the back sprite from Platinum to match the HGSS color change:
Since HGSS, Gengar's color and back spikes has been consistent. But I can't think of any other Pokemon where they kept altering the design to this extent during the first four generations. There were obviously some tweaks and changes during the first two gens, but I feel like just about every other Pokemon had their design "settled" by gen 3. Mewtwo, Pikachu, and a few others had changed fairly drastically by gen 3 but remained consistent after that. Gengar kept getting changes up until HeartGold/SoulSilver!
Just a normal fat ghost, though with white eyes. The Red/Green artwork mostly resembles the Gengar we know today:
...then when they redrew the Pokemon for Blue, they made Gengar, well, blue for some reason:
By the time the anime rolled around, Gengar was back to purple:
And for Yellow, Gengar's sprite was redone to bring it closer to the anime, like all the other sprites in that game:
Gengar remains pretty consistently purple, but of different shades. Compare their Gold/Silver sprites to their Crystal sprites, which is slightly darker:
In gen 3 the purple becomes much more saturated:
Then, interestingly, gen 3 began a trend where Gengar's back spikes start getting drawn differently on back sprites. Only a small handful of Pokemon in gen 3 had different back sprites between Ruby/Sapphire/Emerald and FireRed/LeafGreen:
The back sprite would continue to have disputes on how to draw the spikes into gen 4:
Again, very few Pokemon had redrawn back sprites between Diamond/Pearl and Platinum, so clearly this was notable enough for the sprite artists to take a stand on. Ever since Platinum, Gengar's back spikes have pretty consistently take on the "mesh" look and lost the pointed needles from previous sprites (though they're much less pronounced in gen 6):
...but I must again point out that they changed Gengar's color again in gen 4. Compare the shade of purple used in DPP to HGSS:
They also recolored the back sprite from Platinum to match the HGSS color change:
Since HGSS, Gengar's color and back spikes has been consistent. But I can't think of any other Pokemon where they kept altering the design to this extent during the first four generations. There were obviously some tweaks and changes during the first two gens, but I feel like just about every other Pokemon had their design "settled" by gen 3. Mewtwo, Pikachu, and a few others had changed fairly drastically by gen 3 but remained consistent after that. Gengar kept getting changes up until HeartGold/SoulSilver!
