The current known members of the Mutanimals are Hob (leader), Bandit, Herman, Pete, Puggle, Ray and Sally. Diamond, Lindsey, Mondo, Seymour and Slash are no longer part of the organization. Leatherhead and Mona were never members. And Rockwell isn't...

...Oh, you meant in the 2012 TV series. That show left so many plot threads dangling that it became a lost cause hoping they would be given closure. Some call that the Chris Carter effect.

Anyway, Rockwell and Pete were members, but the third character you mentioned was never actually established to have joined—another dangling plot thread.

Not to mention, they never explain just what happened to Pigeon Pete that made it so they decided to not talk about him.

Yeah, it was this and many other moments that made the 2012 TMNT cartoon infamous for having so many unfinished plot threads.

You know, it's interesting to compare Pete (2012 TV series) with Pete (IDW). The latter character was inspired by the former character, one of perhaps the only ways 2K12 influenced IDW. And yet how their respective productions ended up treating their versions of Pete is polar opposite.

• 2K12 Pete is good-natured, but annoying. Then the show not only dropped him without fate or explanation but had the audacity to trample on him in his absence by making his friends appear to resent him.
• IDW Pete is annoying, but good-natured. The comic never fails to have Pete showing random acts of kindness and caring, even when he can otherwise annoy his friends. But both they and the writers consistently keep him around.

The two versions never seem so different from each other, personality-wise, that their overall treatment by other characters should realistically be all that different. In fact, if anything, Slash (2012 TV series) is an even kinder boss than Old Hob (IDW). But audiences and writers are people, too, and where IDW embraced and loved Pete, 2K12 cursed and disposed of him. Pete seems like a useful gauge for measuring other people's value for and compassion towards people with developmental delays. If this is the case, 2K12 ultimately failed this test, while IDW passed it.

What I'm asking is Where's Rockwell? His final appearance is when he and his friends were at April's Farm House for Splinter's funeral

Yet another unanswered question.

Also, is it just me, or does anybody else imagine Pigeon Pete of the IDW universe to sound like Bobcat Goldthwait?

It's not just that the show kept dropping plot threads. It's also that it periodically seemed to forget or retcon plot threads it was already carrying and write new plots that weren't all that consistent with earlier ones. That can happen with television writing, where writers haven't always seen the entire previous series they're writing for, where only the previous two years of story is officially considered relevant for gauging target audiences, and showrunners can change course on a dime in an even smaller time frame, and the powers-that-be that make the executive decisions may want to retool or discard elements at their whim and instruct the writers accordingly. So for any plot point that wasn't resolved, you may just have to use your imagination, because there's no guarantee at a given point that a production even cares anymore.

So, lacking a definitive answer, what do you think happened to Rockwell? Note there's more than one story continuity version of him: Tyler Rockwell (2012 TV series) and Tyler Rockwell (Amazing Adventures). The latter version appeared in the final page of the final issue of Amazing Adventures, along with fellow Mutanimals Slash, Pete, Leatherhead, Muckman and Mondo, and a Splinter who was very much still alive and was never murdered, wrapping up a story arc (Robotanimals!) where Baxter Stockman had been killed off and was never retromutated. I think AA had its own flaws, having even less storytelling time to work with by virtue of being a comic book, but ultimately I better liked how it used that limited storytelling time than how the TV series ended up using theirs.