Simple as it sounds.
"Simple" would mean: A fox with only one texture, that you can spawn via the spawn command, that just stands there and does nothing.
A lot of mods I use haven't updated past 1.12.2, and I miss having foxes in the game.
With Minecraft 1.13 a lot has changed. This can also be seen, as the Forge project took a long time to move to 1.13 and beyond. So adding foxes (easiest case: copying the code from 1.14 to 1.12) is just as hard as updating a mod to the versions beyond 1.12.
I am aware that there is a fox mod for 1.12.2, but I prefer the vanilla fox model, and the jumping, holding items in their mouths, etc.
But probably the fox mod is your best option. If somebody copies the vanilla fox model, he probably won't be able to publish his mod, as it contains at least the model and textures owned by Mojang.
And to the bigger reason: WHY should somebody invest time to put the foxes into Minecraft 1.12, when he could instead invest the time to help updating 1.12 mods to newer Minecraft versions. Updating the mods is the future, as at some point in time 1.12 will not be used anymore, much like 1.7.10 is nowadays not used anymore for new modpacks. And at that point, the mod will become practically useless.
So if you want your fox mod, you will probably need to develop this "simple" mod by yourself. Or go ahead and contribute to the 1.12 mods, so that they can move to 1.13 and beyond.