I've found a prompt to create to umamusume morphs!
You could do this with other characters but I haven't tried with anything other than umas.
Prompt: "Make the anime woman as fat as the cartoon character. Exactly the same size, proportions, anatomy, etc as the cartoon character. Maintain the face and art style of the anime woman in the end result."
Compatible artist:
Idle-minded
Skelesoda
Huwom_XL
Batspid2
Bamboo ale (theory)
These are the artworks that work best with the original images of the umas (obtained from the fandom wiki) and the prompt. Artwork from artists like "darkfireballz" does not work well since the AI can not comprehend what the hell is going on in the image.
Notes: if you get the "nooo muh guidlines :(" just start another conversation and retry.
If the end result looks like crap, try 2 or 3 more tries and more often than not it should work.
The art must not show any nudity (nipples, vaginas, buttholes, etc.) and sometimes if the art shows "too much skin", like, only having a bra or a full view of a butt in a bikini, you'll surely get the guidlines message.
Sometimes the guidlines message is triggered because of the umas outfit, I tried doing a Mejiro morph but couldn't even after 6 tries with images that got good results. I suspect its because of her outfit but I haven't tried with the flat umas (Haru, Mayano, Tamamo, etc).
Try other characters if you want and enjoy!