Yeah.. could be a data folder or a problem with your Data.ini
But odds are that if you can connect to the game then it isn't a data folder or data.ini problem.
Data.ini should look like this if you open it:
[Data]
0=heRO.grf
2=sdata.grf
3=data.grf
You most likely just don't have the files.
Do you have any .rgz files in your RO folder?
Angel and Mutant got new look a long long time before the trout and bass got their new colors. One example of how you could have missed this patch would be if you never patched when those fish were changed and waited soo long that the GMs took that patch off of the patchlist. The next time you patched it would skip that one.
However, you said the problem started 3 days ago which points toward something with the more recent trout and bass colors. This shouldn't affect Angel or Mutant unless something was done in the more recent patch to change them. It could have been corrupted or something while downloading to you. Or you could have quit the patcher before it was done. Either way just download this file and place it in the folder you have heRO installed into.
Link will show up once it's done uploading.
Fully Updated heRO.grf:
Here