I know this isn't really speedrun related and it'll probably get removed, but I've searched far and wide around the internet and I can't find ANYTHING about this game. So, if anyone has any information on the history or development of the game or any secrets hidden in the code, I would appreciate it more than anything. Thank you!

Hello! I and a group of other people have actually been doing a bit of research on this lately (and one of us is planning on making a video on it as well). Yeah, history on it is pretty hard to find, but we have actually made some progress on this, so I would be happy to share some with you!

This is the main discord server we've been discussing our findings in, which may be a bit more helpful than whatever i can toss together at 3am. I probably missed some stuff and if we find anything else then we talk about it here first
https://discord.gg/EBhSvf73HU

Here's an old website for the game, a dog version and a cat version. It's all in Japanese, but even if you don't know any you can still find some interesting stuff looking around these:
https://www.ubisoft.co.jp/dogcat/dog/chara/index.html
https://www.ubisoft.co.jp/dogcat/cat/chara/index.html
One of the pages includes a trailer which requires Flash to view, but I got a friend to record them shortly before it shut down:
cat version https://t.co/R1chTH1lN4
dog version https://t.co/5qw0CeFADZ

Something of note that can be found across the website and trailer are two earlier versions of pawville, one of which is actually also visible in the credits of the final game if you look closely. Also I will never get over the fact that there was going to be a vulture on dolphin coast

A few months back, I tried datamining the game and found a handful of unused sounds and icons for things (eg. the magic hat has 4 "item" icons as if it were going to be a special item, the 4 being the different stages from its normal form to pure evil). A while after that someone came along who tried modding it, and he discovered some stuff of his own (he was the one who discovered what happens when you head through into Gongoro Peak early)

We've found a group of news websites with screenshots from before the game released. Here is one example:
https://www.nintendo-master.com/news/screens-dogz
Said screenshots mostly feature areas from THE DOG Island (Green Meadows, Zoomy Lake), as well as one of the earlier Pawville designs and a slightly more bare-looking version of Jade Fields. Also, each one has an identical minimap displayed. We have about 6 each for Dogz and Catz, and we found a french website that seemed to have more but it's currently dead and the images aren't saved on the wayback machine, just the article they're in, so for now they might be lost.
The state of the screenshots also seems to imply they were taken surprisingly early in development

Hope this helps you