Something that bugs me is how Gen 3 went through 3 stages graphically apparently for mons, yet remnants of earlier stages still pop up randomly, creating inconsistencies
The following are theories!
So we'll start with the first stage with one of the earliest designed Hoenn mons, Wailmer 2001 Corocoro
In this stage the first 9 mons were shown. Mons had no black in outlines, and shading was very banded for front sprites. Colors also were more saturated, and a good chunk of battle effects were done (except Surf and Reflect)
Incidentally, this period was also when the berryshop was implemented. And noticeably in the final berry screen...
There's no black outlines! Though these were most likely done at a later stage compared to 2001 battle effects given more muted colors
Which we'll bring up on. The later stage was probably when GF noticed that the GBA screen....was really bad
So focus shifted to the GC player. Some mons were updated to the same artstyle until it changed again, and final incidentally still had some of them use that style.
Others include:
Manectric back, Surskit, Masquerain back, Seedot, Baltoy, Claydol, and Duskclops back
I would estimate this style was kept for a while until late 2001/early 2002, though the lack of visuals in leaks don't help
Note: Player Trainer sprites seem to have been finished early. They barely changed in dev
And then ignoring sprite shading styles, you have remnants of earlier mon designs in sprites compared to art (also including Pinball)
-Delcatty having a large dot on its head
-Nosepass
-Blue Salamence
-Metang having notches for back
-Walrein's huge nose
-Electrike's missing spikes for back
-Linoone's tail being properly connected instead of looking like a flat sheet
And then despite GSC having the Gen 1 retcons, many still were in RS or RS Pinball
-White belly Weedle
-4 fingered Mr Mime, Haunter, and Poliwhirl
-Nidoqueen fangs
-brown fingered Graveler with grey body for Pinball
And Venomoth potentially if its palette indicates the yellow spots were planned
And then there's the fact that OW sprites were based heavily on Gen 2 Trainer sprites, then were never updated....
Gen 3 is a mess, yet I love it