I know there's already an Anthro Character replacer but IDK how it really works? As for games that allow custom portraits, I just found out Mechwarrior 5 has it.

I'm not exactly familiar with how to make mods from scratch, unfortunately, but this might help give you a starting point: https://stardewvalleywiki.com/Modding:NPC_data

There are a couple mods to replace your character and village NPCs with anthros. Their portraits are also updated to reflect their new looks too. It's just some setup and having SMAPI installed. That being said, it's been a long while since I last played since I too am waiting for 1.6, so I'd have to recheck the setup process.

There's a Pokémon mod for Stardew Valley.
Just sayin'. xD

As for the custom portrait games, There's Blazing Beaks, Deadnaut, Monsters' Den: Godfall, Icewind Dale, Neverwinter Night, Wizardry 8 and... That's all I remember. xD

The Pathfinder games allow custom portraits. I'd always use some of my OC's for them, even if they made no sense.

I've dabbled in Stardew modding a bit and it's as simple as editing a character sprite sheet and using Content Patcher to make sure it only replaces the coordinates of the image you actually changed. The game has a lot of animations though so editing all the sprites is tedious. If you need guidance you can download a mod that replaces a character and see how they did it

Alternatively there is an anthro character customization mod you can use but it relies on mods that are outdated so you'll probably have to mess around with some files to get it to work

And for games with custom portraits, most CRPGs allow them

I don't know the mod making process, but the game just recently updated, so modding might be in a bit of an upheaval.

Are you making a mod for SV ?

i remember sort of making some cow re-textures, it was pretty easy all things considered, but it has been years id have to refresh, but hey if i can pull it off it cant be too hard