Afraid or Frightened?

You don’t use afraid before a noun. You can say a frightened animal, but not an afraid animal. 

You can be ‘afraid of ‘something and you can be ‘frightened of’ something:
I’m afraid of heights.
She’s frightened of getting old.

You can also say that you are ‘frightened to’ or ‘afraid to’ do something:
I’m afraid to speak up.
People are frightened to leave their doors unlocked.

Afraid can be used to politely express regret, like this:
I’m afraid that we don’t stock that item.

But you don’t use ‘frightened’ to express regret.