I noticed that on Wikipedia's page for Taz-Mania, it's mentioned that although Wendell T. Wolf was mentioned as a principal character in the opening, his appearances were few and far between. However, I don't remember the bush rats or the kiwi appearing in very many episodes, either, and they were in the opening, either. Wikipedia doesn't mention what it means by Wendell being referred to as a principal character in the opening (my only guess is due to him being the secodn character mentioned, besides the tasmanian devil family). It was also mentioned that the episodes with Wendell had inside jokes about the fact that he was in the opening but rarely on the show. I wonder what these inside jokes are. I saw two episodes with Wendell on in2tv.com, and didn't notice any such jokes.
But it seems like it was common for such jokes to be made about other characters. Wikipedia mentiosn that Buddy Boar made less appearances in the later episodes, and one later episode had a joke about this, stating that nobody liked Buddy so he became a producer on the show. In the episode Taz Like Dingo, Mr. Thickly made a cameo appearance as a dragon, mentioning that he's made that cameo because he hadn't been on the show much lately. So I guess that means that Mr. Thickly appeared less in the later episodes. And I've seen one episode that has a scene taking place behind-the-scenes, in which Hugh makes a cameo, saying "I bet you're wondering why I'm not in this episode".
in2tv.com has quite a few episodes, though I think they're mostly from the first season. I haven't watched every episode that's included there, but I haven't been able to find any episodes with Constance, Mum, the Bushrats, or Willie Wombat. I wonder i any epsidoes are available on youtube.