It's a Boo Steam that moves up/down with Left/Right Wave motion.
On Extra Bit Set it'll initially start moving right, otherwise it initially moves left.
It also uses first property byte for custom intitial Y speed (By default, it "Faces" Mario). More info in ASM.
Credit is optional.
Technically, the boos move in a paraboloid motion with the direction changing at the maximum spped instead of a true sine wave but since it's a good estimation, it does the job properly.
Other then that, it's basically a modified boo stream with some tables moved around, defines added or removed, the stated movement change and the fact that you can chose the initial Y speed.
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