Oops, sorry to double post, I'm just going through all the unanswered posts on the forum backwards and didn't realize the question about the " bugged out texture in the Castor Wild tileset" was the same topic hahaha
The tilesets in this pack look entirely unfinished and kind unusable in their current state. It looks like someone just overlaid a 16x16 grid, and made everything traversable. There's also tons of "tiles" over the transparent, unused parts of the png. I guess if you're using this though, you probably deleted the tilesets and redid them from the source .png file though?
But if you're asking about the pink, I have a guess for what that is. Because that's the exact shade of pink that everyone used to use back in the RPG Maker 2000 days hahahaha.
Back in those days, RPG Maker didn't read transparency in files. So you'd choose one color to be the "transparent" color, and RPG Maker would treat and tiles that contained that color as if that color was transparent. So you'd need to choose a color that wasn't contained anywhere in the actual tiles, because if you used like white for example, any tiles that were white (snow, highlights, etc) would have "holes" where the white was. Or if you used a shade of green, you'd risk parts of your trees going transparent or something. It might have also been limited to 256 colors? Can't remember. But anyway, basically everyone settled on that shade of pink to be used as the transparent color, since it's basically never used in actual tiles. I'm guessing that was carried on out of habit and used as a placeholder for transparency.
But then looking closer, there's all kinds of weird stuff going on. Some of the tiles have like, a pink overlay, or there's some places where water tiles look to have turned grey somehow. So I think in addition to using the pink for a placeholder, there was also some recoloring mistakes. The tilesets also look like they have a bunch of garbage tiles that were leftover from ripping the graphics from the GBA cart. Like all those little boxes in the Hyrule Town tileset. So these were never cleaned up from the initial ripped graphics, and there could have been some problems during that process that corrupted some of the tiles as well.
TL;DR, I'm guessing that the pink in the tileset was used as a placeholder for transparency, or was left from old Minish Cap rips and wasn't removed. And then it looks like when the tiles were ripped it didn't work perfectly, or there were some mistakes when recoloring. But I don't know how you're using these tilesets anyway because as they are in the repo I just downloaded, they totally need to be redone.
Side note, if you are redoing these or want to and could use help, hit me up. Preferably on the discord server so I'll actually see it haha