Battle for Mata Nui was a Macromedia Shockwave game released during 2002. In this real-time isometric strategy game, you control the Matoran, Toa, and Turaga of Le-, Ta-, and Ga-Koro as they try to fend off the Bohrok swarms.
As a kid, the sprites in BfMN looked odd to me, but the only thing I noticed was that Tahu has Vakama's firestaff instead of his normal fire sword. However, when I ripped the assets from the game recently, I gave the sprites a closer look, and noticed all sorts of interesting things:
(special thanks to r543 for noticing the slizer-like torsos.)
From the number of Slizer elements, especially considering that the Boneheads of Voodoo Island used Slizer heads as feet, I think these sprites are based off of undocumented prototype 3D models. How those pre-2001 models ended up being used as reference for a 2002 game is a mystery, though. My best guess is that when Lego commissioned this game, they sent the game devs a bunch of miscellaneous assets without vetting them too carefully. (This could also explain why some elements are reused from the 2001 Bionicle GBA game.)
Once I finish ripping the Bohrok sprites and the background tiles, I plan on submitting all the assets to Spriters' Resource. Until then, here are the Toa, Turaga, and Matoran: https://imgur.com/a/fJIwXuq