It can be an original from you, or a takeoff on an existing one. You can reflect on the success or failure of your team with your lyrics. Here's mine for the Buffalo Bills, a team entrenched in suckiness. Their theme is the Isley Brother's "Shout". I have parodied it thusly:
The Bills make me wanna
(s**t)
Hit the restroom and
(s**t)
Pull my cheeks back and
(s**t)
Pinch a loaf off and
(s**t)
Yell nothing but
(s**t, s**t, s**t, s**t, s**t, s**t, s**t, s**t)
(s**t)
Chan Gailey's got this team going in reverse
(s**t)
Fitpatrick stinks the offense up, the D is even worse
(s**t)
13 years of playoff hopes are goin' down the drain
(s**t)
Buffalo sucks so bad it's drivin' me insane
(s**t)
Heyyyyyyyy
(Heyyyyyyyy)
Heyyyyyyyy
(Heyyyyyyyy)
You suck, Buffalo
(You suck, Buffalo)
You suck, Buffalo
(You suck, Buffalo)
repeat
Kinda sums up my frustration over the team.