This is the infamous "Ultimate Noob Boss" made by Nesquik Bunny. It is now back with a new name that implies it can be used for more than boss fights. It has quite a few attacks and is very customizable. Read the comments in the .asm file for details.
This one got lots of optimization, and even new functions. Read the readme.txt for more details. Added screenshots, tags, and SA-1 support.
Extra comment: Even nowadays, this sprite has great potential, despite being overused and often badly implemented in hacks. It's an annoying sprite only if you don't know how to design a level with it and its properties.
Probably not a good idea to use it as a non-boss sprite, because if you place more than one in a level, sometimes they can act weird. The most notable issue is regarding animations. If, for example, you're using 2 sprites with animation change states, when one of them is hit to change animation, will also make all other change as well. I don't know if I explained very well, but yeah, I don't recommend using more than one of this sprite in a same level unless you can work around these issues.