Mr. Sax is the Second boss encountered in the original Rayman game; Mr. Sax is seemingly in charge of Band Land during Mr. Dark's coup. Like the other inhabitants of Band Land (save for the cricket enemies), Mr. Sax is a sentient instrument, in this case a gigantic saxophone.
He is first seen at the last level of Allegro Presto, but runs off before Rayman can actually do anything. Mr. Sax is not fought in a formal boss fight until Mr. Sax's Hullabaloo. After beating the first area here Mr. Sax confronts Rayman proper for the first time, but the fight quickly turns into a chase through Band Land with Mr. Sax trying to crush Rayman. At this point Rayman has a chance to knock the occasional Purple Note into Sax and damage him; damage from this chase results in Mr. Sax starting with a lower health bar in the subsequent fight stage. The notes are the only way to damage Mr. Sax.
Once the chase has run its course, Rayman will fall into a pit and confront Mr. Sax in a final battle. Here, Sax will fire both Purple Notes and Bomb Notes; the latter of which explode in mid-air and release Mini-Notes (which cannot be knocked back). Mr. Sax has no other tactics beyond firing these projectiles and jumping across the arena when hit, and his lethargic movements make him a fairly simple boss encounter.
After Rayman defeats him, Mr. Sax collapses and then beings dancing deliriously, but he is presumably still considered "defeated". At the Candy Chateau showdown with Mr. Dark, a saxophone is seen painted on a glass window, which represents Mr. Sax being Mr. Dark's minion. When Mr. Dark morphed all of the bosses together, Mr. Sax was merged with Moskito and Space Mama.
A song called "Mr. Saxobeat" was released by Alexandra Stan in 2010, as it almost relates to Mr. Sax himself. Though the song does not talk about Mr. Sax, the name of it almost sounds like Mr. Sax from Rayman 1.