iPhone 7+ (iOS 11.1) owner here. If a friend buys an iPhone X and creates/sends me an Animoji, how will it appear on my phone (if at all)?
Short answer: Yes.
According to Apple:
You can create your own Animoji and share it with anyone using an iOS device, Mac, or smartphone.
The iPhone X can create the Animoji which can then share the animation to anyone running iOS, macOS, or a smartphone - ANY smartphone. See this article for more details
The Animoji is saved as a
.mov file which is sent via MMS and can be viewed by virtually any smartphone (not just iPhones).
Yes. This site states that Animoji can be seen and heard on other devices because it's sent via MMS (Multimedia Messaging Service).
In fact, people with an iPhone X can save their Animoji as a .mov file or as a sticker to use in other messages.
I can verify this works. I sent an Animoji to my wife who has an iPhone SE. She was able to see and hear the message.