So, it's a simple question. Is there any rule against having a summon that is humanoid? This might seem weird, but I was trying to brainstorm certain concepts to increase diversity in my characters, and a new type of summon was one that I came up with. Looking through the different beasts that were claimed, I see things like Golems, Oni, and apes. All of these can be custom made to appear human, with the qualities of their particular class. The most prevalent example would be monkeys. Enma is essentially just a regular ninja with a lot of hair and a tail.
So would anyone care if someone made - oh lets say - merfolk? I realize that there are individuals and techniques that create what may as well be merfolk already (Kisame is the prime example) but that's why I'm just using this as a rough outline. Most summons at base are sophisticated enough to have entire cultures, like the toads or whatever custom animals people have come up with. So would it be that big of a deal to create a summon that while shaped like a human, retain qualities that clearly make them different?