After spending some time coming up with an idea for a team mascot, it makes sense to flesh out the back story and really bring your mascot to life.  This is a lot more fun than most people think, since you can include all sorts of elements to make the story interesting.  Here are some examples of things to include in your team mascot’s unique biography:
Birthplace and a Funny Story
It never hurts to begin with the place where your mascot was born.  Come up with a colorful name for the hospital and town where the event occurred.  You may even want to add some sort of unusual circumstances surrounding the birth, like mistaken identity.  Perhaps your team mascot is in the design of a biscuit, and there was momentary confusion in the maternity ward between the biscuit and an English muffin born the same day.  It took a toasty nurse to figure out what was wrong and make things right, much to the relief of the parents.
A Family Tree
Involve some information about your mascot character’s family as part of the back story.  Perhaps the father and mother had colorful careers before the little team mascot came along.  Papa may have worked in a factory that made sports equipment, while Mama sold popcorn at the games.  One day they met and it was love at first sight.
There could be a relative or two that is only talked about in hushed tones, and only seems to appear at the most inconvenient times – like an eccentric uncle or flamboyant cousin.  Along with making the mascot’s past more interesting, it also paves the way for a visit from one of those colourful relatives in the future.
A Romantic Interest
It never hurts to spice things up a little by having some sort of romantic interest for the team mascot.  Perhaps there is someone who is in constant pursuit but the mascot is determined to dodge them.  This sort of “will they or won’t they” tension can last for seasons before it has to be resolved.
The Mascot’s Achilles Heel
There has to be something that strikes fear into the heart of the mascot.  Come up with something that has to do with the nature of the character.  For example, if the mascot happens to be an elephant, spotting a mouse causes the mascot to jump into the arms of the nearest player. Remember that a little performance training will help the person in the suit develop a response that delights the audience.
How The Connection Came to Be
Think of a fun way that the team and the mascot connected.  Maybe a talent scout spotted the mascot being chased by pack of hungry dogs and saved the poor fellow from oblivion.  Perhaps the mascot wandered into the stadium one day, tried out for the team, and was awful.  Even so, the team decided to keep him around.
As the fans know your team mascot, the more they will enjoy it when the character takes the field.  Embellish the character with new traits as time goes on.  Consider giving the character a makeover now and then, which would explain why the custom costume design changes every few seasons.
Above all, have fun with the character development.  There is even the possibility of getting the fans involved by holding contests to see if the mascot will get a new friend or take up some hobby.  Once the character has a good back story, the sky’s the limit in terms of where it will go.