Minecraft wiki entry for Phantom, associated text: At one point before the release of 18w10a, the phantom texture was made red, but it was decided that it appeared too much like a Nether mob.[4]
The phantom was originally going to have a mouth, like the drawing shown during MINECON Earth, but it was eventually scrapped because "it looked so much like a muppet".[4] Earlier designs can be seen on Minecraft's Instagram account.[5]
The sounds of the phantom were taken from Samuel Åberg's baby son screaming.[6]