Hallie is the Harmon's family dog, a poodle/chihuahua mix or poohuahua. She is a character in the first season (dubbed by fans "Murder House") portrayed by "Lambchop". Hallie can sense the ghosts and will bark at them. She barks at the basement when the Harmons first look at the house. Ben accuses Vivien of having bought a dog instead of investing time in rebuilding their relationship, putting her affection towards the dog instead of towards him.
Hallie went down into the basement with Violet and began barking at what is most likely the Infantata, though Violet can't see him. 
Hayden pretends to kill the dog in the microwave. Vivien finds Hallie later on in a closet. In reality, Hayden admits to have put a tomato in the microwave to get the effect she needed. 
Hallie is in (real estate agent) Marcy's arms, wearing a pink outfit, as Marcy shows the house to the Ramos family, telling them about the Harmons' "romantic tragedy" for the sake of full disclosure and stating that afterwards she adopted Hallie. 
She was mentioned a few times in episodes of Hotel. In the first episode, Marcy announced that she was feeling bad because she had to put down the dog, that she got due unexpected circumstances. Also, some episodes later, she was packing her stuff while talking with a friend on the phone when she said that she was going through a bad period because of her dog's death.