What kind of animal is a "masi"?

Mum-mum and Vyom decided to play the game of categories the other day.

Mum-mum: "Categories, if you please, name some, names of... Animals
for example, Dog..."
Vyom: " Billi-myaaoo"
m-m: "Donkey"
V: "Elephant"
.....
m-m: "tota-ram"
V: "mmmmm.... Doncy masi"
m-m: "Doncy masi is not an animal"
V: "hmmm... Pinka masi?"
m-m: "No, Pinka masi is also not an animal"
V: " Purva masi!" (half-screaming in triumph)
m-m: "No Vyom, masis are not animals. Masis are girls, see these are animals"
V (frowning and silently looking at the pictures, and then saying slowly but very firmly) " Masi, Animals, YES, Okay?"
(mum-mum moved to non-controversial categories like colours)