You’re experiencing z-fighting. Here’s a quick video demonstrating the effect and an explanation on Wikipedia
In essence, this happens a lot in 3D graphics when 2 planes occupy the same space, so the renderer doesn’t know which plane is in front of the other, and it flickers like that.
Just make sure the base of the dolphins isn’t being duplicated when you’re importing them. Also, since you don’t know where you downloaded the dolphin assets from, it might be an issue with the assets and not your code. It’s hard to tell without a code sample. It has nothing to do with ambient light, and changing the near parameter won’t fix it.