Baxter, Rat King and Leatherhead
 

Did anyone else notice that after Shredder/Krang/Bebop/Rocksteady, the writers tended to use Baxter, Rat King and Leatherhead as the next main villains?

I suppose Slash would count too as he did get 3 episodes, although he hardly did anything in "Night of the rouges" besides throw a pizza parlor on top of Michaelangelo.

I find it interesting that all 3 of them were Mirage created characters too, and not Playmates/Fred Wolf created characters like some of the others were, such as Groundchuck/Dirtbag, etc.

Yeah, you can say that, but 2 of the 3 of the are so radically different from their Mirage counterparts, that they may as well have been Fred Wolf creations.

You can say that for any character though. Shredder, April, Splinter...everyone was different.

True, but Leatherhead and Baxter were VERY different. Shredder for the most part was the same. April and Splinter do get a radically different interpretation, as well, but Leatherhead becomes a Cajun villain and Baxter a goofy white stereotype of a scientist..then a fly.

Rat King isn't all that far off from his Mirage counterpart. At least when compared to others.

Now that you mention it, they seem like they were the main villains (except in Seasons 9 and 10, that was Lord Dregg). Even if they're not Shredder and Krang, all 3 of them have worked with them at some point in the cartoon. Without Krang's mutagen, Leatherhead wouldn't be who he is.

True, but I believe Big Louie worked for Shredder in at least a few episodes, didn't he? The one with the cufflings and I'm thinking of another one.

The other episode is "Napoleon Bonafrog: Colossus of the Swamps".

I think another reason that RK/LH/BS were the other main villians and were as popular as they were is because they were introduced early on in the series, so fans were able to kinda latch onto them early on in the series. Not alot of memorable villains were introduced later on in the series (except the occasional ones like Groundchuck and Dirtbag and Lord Dregg).

That is true, by Season 3 all of the shows most notable characters were introduced, including Casey Jones.

Seasons 4-7 basically introduced second tier heroes/villains, some of which were good, but wouldn't be as important.