Can't you just call draper in your partial #{start.horse.decorate.boy_or_girl}"
?
If you want to call the decorator only in the controller, you should use decorates_association
I guess you will have something like
class StartDecorator < Draper::Base
decorates :start
decorates_association :horses
...
end
class HorseDecorator < Draper::Base
decorates :horse
def boy_or_girl
# your code
end
...
end
See also this question