Chicken can actually be both pollo and gallina

chicken SUSTANTIVO

1. (animal)
   a. el pollo (m)
   They had chickens at the farm.Tenían pollos en la granja.
   b. la gallina (f)
   The chickens were incubating their eggs.Las gallinas estaban empollando sus huevos.
   source

I appreciate the comment @danilamarilu, but please also check before saying it'a huge mistake. I will add pollo above. Thanks!

Edit: hen can be used for gallina, as well as chicken.