How are the other enemies "realistic"? Many of them are all based on Amano's drawings, mythological beasts of some sort.
Well aside from the goblins which just look like imps, many enemies are high-tec knights, robots or actual beasts, such as the behemoth and the big yak-like beasts grazing in the fields.
The naga is a human head with jewelry on the body of a snake. It does look more "fantasy" than the other creature designs.
It could be pulled off great in an area that is more ancient or "forbidden", but it simply looked a bit odd out there in the middle of the field, that's all.