Are all primary prototypings just heads?

Yes and No. Generally, Sprites are just icons until they enter the Medium, at which point they Split. Two-Tier Prototyping before Entry seems to consistently make the Sprite take its final form even before splitting. And on top of that, some Tier-1 Prototypings Pre-Entry have been shown to be more than just a head- With Aradia’s Frogsprite seeming to be the full Sprite inside of the Kernel, and Bec got the tail to fly up because, I believe Hussie mentioned, “It’d look just goofy if it was just a Bec Head floating up and firing a laser beam.” Additionally, some Kernels may get peripheral stuff- For example Harlequinsprite’s Arm was hovering slightly outside of the Kernel, and Seppucrow was able to peck Dave with its beak, and the Katana was similarly peeking out, so like, Sprites are pretty flexible when it comes to their form and interactability.