Really more of a WCIF question than a game help query.
The chicken mascot outfit was a giveaway provided with pre-orders of OFB before it was originally released.
It is just an outfit, and can be worn like any other for whatever situation you desire.
Available for download at MATY (no registration required):
http://www.moreawesomethanyou.com/smf/index.php?PHPSESSID=f7bedb2c159625b9d4df4e0e2f48f619&topic=11573.0To view and access the 'hidden,' NPC and other such outfits, hairs and skins in C-A-S, enable testing cheat when in neighborhood view:
boolprop testingCheatsEnabled true
then enter C-A-S and press Shift-N while in C-A-S, before creating a Sim. Shift-M re-hides those outfits.
Turn off testing cheat:
boolprop testingCheatsEnabled false
when done.