This puzzle is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.
If a word conforms to a special rule, I call it a Multietude™ word.
Here is a list of Multietude™ and not-Multietude™ words:
And CSV version:
Multietude;Not-Multietude
Spy;Infiltrator
Baby;Boy
Mouse;Cat
Louse;Beetle
Penny;Clay
Domino;Card
Puppy;Hay
QUESTION: What is the rule to tell whether a word is a Multietude™ word or not?