Not only is it from 4 different zone/biomes, they are almost all from different updates so they have no reason to be grouped up like this.
So if i had to guess, i hope that in this case that doggie actually isn't this disorganized (Wouldn't be shocked if it was the case) and the reason it's this bad is because they actually don't use multiple sprite sheets but one giant one. Now the one giant sprite sheet idea has some ground, multiple sprite sheets have no relation in what they contain, the sprite sheets contain items from the first enemy releases of the game to the cursed ABDL mint.
Now in terms of coding potentially the giant sprite sheet gets uploaded and everything gets broken down from there and assigned but the game engine breaks it down further for easier file read times.
Or doggie could be this disorganized.
Anyways here's the sprite sheet labeled 1
View attachment 4799085
Some of it makes sense as they should be loaded first but then you get enemy sprites that aren't even apart of the same region / updates so perhaps the dev just uses a really oversized sheet and then it gets broken down by the game engine for the release we get. Which hopefully explains how fucked these sheets are. (ABDL has been censored because mint is in that grey area i don't wanna fuck with / am not comfortable with)