It's only been a few weeks since Duo added three fun Scooby-Doo effects for its video messages, but that's not reason enough for the team to rest on its laurels. In order to earn their candy-filled stockings and gifts under the tree, they had to give us a special holiday-themed effect and now it's live in the app.
Open Duo, swipe down on the homescreen to go to the video message recording screen, and tap the Effects icon on the right. The first option you'll see should be a pair of deer antlers. This is the effect you're looking for. It straps deer antlers and a mistletoe on the top of your head, with deer ears too, a nose, and some brown spots on your face. You'll also see snowflakes falling and an overall white haze all around you. If you like the effect, you can record a message and send it to your Duo contacts right away.
The effect appears to be triggered via a server-side switch. My Pixel 2 XL and 4 XL are both running v67, but the former doesn't have it while the latter does.