There may have been a "real" Mijumaru.
So we have an approximation of the name "Mijumaru", breaking down miju and maru as water (mizu), naive or immature (mijuku), round (maru), or the common boy-name suffix (-maru).
But say this: let's assume it's meant to be "water" and "round". Why?
I discovered this accidentally by going through TVTrope's page on "Better with Otters" or close to that, some while back. Ring of Bright Water is an autobiography about a man's life, including his time with an adopted otter, named Mijbil.
Mijbil. Who they nicknamed Mij. Phonetically, Mij would be Miju (while Mijbil would be Mijubiru).
Now take into consideration of why "water" and "round" by looking at the title of the book: Ring of Bright Water.
I figured you might get a kick out of this (since it wasn't acknowledged on BP when I brought it up).
O.O What The Shell. Thats.... Interesting....