Apparently there's a Danish cartoon character that is a chicken - Rasmussen being so skinny has been compared to the cartoon character (I've also heard him called skinny chicken and chicken legs...)
I've heard that nicknaming in the peleton is pretty common and you don't get to choose your own nickname... I think is more rare that we, the spectators, get to know what the nicknames are.

Rasmussen dancing on the pedals - funky chicken
Rasmussen's TT performace last year -........ chicken (I'll let you guess )

oh - as far as nicknames go - you know Manuel Beltran, even the commentators call him Tricky sometimes. Well its not because he is a good tactian. Apparently Tricky is the Spanish Sesame Street name for Cookie Monster - seems Beltran has a sweet tooth....