Why is it called a chicken bus? Because people have transported EVERYTHING on these buses, even chickens. They are originally school buses from the US that have been retired and driven down to Guatemala. Then they are reconditioned and redecorated. They get filled to the gills on top and inside and transport people and stuff (including chickens) from city to city. The drivers have a reputation of speeding even on twisting roads.
The following photo was taken during a flower festival in Antigua. There were several chicken buses decorated in such a way that people could pose in front of them.