This is yet another one of Nifflas' games that has a sheep hidden somewhere. Specifically, in
Level 31, the level that introduces the pulses that mess with your ship's abilities.
As a few Miiverse posts I archived (really need to get around to uploading those) can attest to, people already found it years ago, but not in the intended way. That same level turned out to have a glitch that allows you to find the sheep through unintended means. When that glitch was patched, people falsely assumed the sheep was now unreachable.
After getting Nifflas' confirmation that there is indeed an official way to get it, I managed to find it. In the unlikely event that anyone else wants to find it themselves, here are the hints he gave me:
#1: You have to go out of bounds through a trick that doesn't involve a gap in a wall.
#2: The trick was originally a glitch during development, but was deemed too difficult to fix. The sheep seems to have been added as a reward for those who manage to trigger it.
And here's the full solution:
Position yourself between the two huge doors at the beginning of the level as they move up. (It won't work if you're to the left or to the right of both.) Then stick to one of them. Though this doesn't always work, most of the time, it'll cause the door to drag you into the wall at the top. A flaw in the game's collision detection will then cause you to go through it. You're now out of bounds, though still stuck between two invisible walls. Fly up until you can get past them, then to the left edge of the level, down and finally to the right again.