I guess this is a:
If I wear a canine head, I’ll become hard like a rock.
This line suggests that when the object (a scarecrow) is equipped with a canine head (resembling a dog), it becomes a hard and intimidating presence in the field, scaring away potential threats like birds or animals.
But if I wear a cervid head, I’ll be almost as soft as a sock.
Here, a cervid head (resembling a deer) makes the object appear less threatening and less effective at scaring pests, thus making it "soft" in terms of its effectiveness.
If I wear a fish tail, I’ll be dead and rotting on a plain.
This line implies that when the object has a fish tail, it is ineffective as a scarecrow, and it fails to deter pests. It is described as "dead and rotting" because it can't fulfill its purpose.
But if I wear a hominid tail, I’ll become lush and green again.
When the object is equipped with a hominid (human-like) tail, it becomes effective at its job, scaring pests away. This is described as making it "lush and green again," indicating that it helps protect the crops and keep them healthy.