I think that this is Scooter making reference to a band from which they take some of their inspiration.
Scooter base quite a lot of their songs on singles released by an 80s Techno group called The KLF - indeed some songs by Scooter have samples lifted directly from KLF singles.
Behind the Cow lifts lyrics and structure from a KLF song called "What Time is Love?" - and KLF were also known as The Justified Ancients of Mu Mu. The KLF song "Justified and Ancient" features the lyrics "Ooooh oh bound for Mu Mu land".
So "the Legend.....behind the cow (i.e. Mu Mu) " is a reference to the band The KLF.
There was also a reference to "The Painted Cow" in the Scooter song Nessaja (which has been used in Combat and Step).
Bet you are glad you asked now eh?!
I'm damned, I'm hot and I just can't stop