Megaman Bass [Mega Man Bass] is a ROM hack created by Falchion22.  This turns the main character from Mega Man to Bass, hence the name, as well as his own story line.  Falchion22 was the first to make Zero playable in Mega Man X titled "Mega Man Z".  As Zero was more of a palette swap of X it is similar here with Bass opposite Mega Man, but it's close enough to feel like you're playing as Bass (at least the buster shots are different and look pretty cool). There are two versions I made available:

Base hack and MSU-1.

With individual Refit patches.

• Faster Charge Shot
  

• Player I-Frames Increase
  

• New Damage Data
  

• No Charge Spark
  

• Untouchable Rush + Shield Fix
  

• Stronger Charge Shot