The animation was great, I loved the custom sprites!
My main issue with this animation is that in terms of plot, it's pretty aimless. You started off with a narrative of the Toad vs. Koopa war, and then you went on to call the dog a hero, but you never showed how the dog became a hero or affected the war in the end. The dog was basically scrambling for dear life the whole time. It seems you were going for 'accidental hero', and that would have worked out fine had there been a resolution, but you ended with the dog just running away - the story wasn't concluded. Don't get me wrong, the way you sequenced things in the scenes was great, it's just that there wasn't much happening plot-wise, and many things were left unexplained and not followed through with (like the Dr. Mario viruses, the way they had no effect later on despite messing with the dog's organs made that whole scene pointless; they ended up making a cameo for the sake of making a cameo).
The story laid down a setting, but never followed through with a plot direction, and as a result much of the impact that could have been reaped from the backstory was wasted. It felt like the characters were standing around much of the time, especially the dog, because of the slow pacing. Not much was happening in real time, and things tended to coincidentally happen one at a time - it wasn't very natural or believable and came off as cobbled-together. It almost seemed like you wanted to make an hour long movie, but cut it short (even then, the pacing would be too slow).
Sorry if I rambled, those were the things I had a problem with, it was just a little difficult to pick them apart precisely so I might not have been all that clear. I did the best I could at the moment, though. Putting aside the issues of plot and pacing, I can tell you put a lot of work into this, and there's a good amount of skill being exhibited here. Your pacing may have been slow, but your sprites and screenplay were artfully done. Overall, it was an interesting story that just trailed off at the end. Perhaps a bit too ambitious for the planning that went into it, but it was interesting nonetheless.
Good work! I hope to see more from you, and good luck on your next project!