I tried to make a video like this myself a while back, but this one is much better and more complete -- as far as I can tell, it's all of Pinky's responses to the "Are You Pondering What I'm Pondering?" question from "Animaniacs" and the spin-off series. I think they did the routine a couple of times on "Pinky, Elmyra and the Brain" and in the "Wakko's Wish" movie, so it's not one hundred percent complete, but it's all the responses from the commercial DVD sets.
They're not arranged in order, which makes it less clear, but there was an evolution in the kind of answers Pinky gave. Originally it was something that implied he was "pondering" a solution to the actual problem he and Brain were dealing with, but that his idea was something weird or potentially obscene. Midway through the spinoff, it changed to become a random thought on popular culture or some other issue that happened to be on Pinky's mind ("But 'Tuesday Weld' isn't a complete sentence").
There was also at least one "pondering" that was written in and redubbed at the last minute. A longtime fan (I think it was Ron "Keeper" O'Dell, the keeper of the most important "Animaniacs" online resource) suggested "I think so, Brain, but she'd never leave Mickey." It was apparently recorded, but the Warners legal department ordered it out because of the knotty compensation issues involved -- you're not supposed to use unsolicited ideas, even if they give you permission. So it was changed to "But then my name would be 'Thumby.'"