Tim Hardwick, MacRumors:

In macOS Monterey, you can use an animated Memoji as your Mac user profile instead of a standard static image. Of course you could always use a still photo of yourself to welcome you at startup and on the Lock Screen, but where’s the fun in that?

And, once you’ve made the change:

Next time you login to your Mac, you’ll see your animated Memoji avatar there to welcome you. And if you get your login password wrong a few times, just watch the expression on their face go from mildly irritated to thoroughly irate.

I followed Tim’s instructions, made the change, can confirm the various “wrong password” interactions. Fun!