Where does the horse fit into the cladogram below. Which phylum is it in?
What adaptations or body features caused the animals to be classified this way?

Chordata.

Explanation:

Chordata is the phylum from which the horse belongs. The presence of  four common features in organisms that appear during developmental stages i.e. These four features i.e. una notocorda, un cordón nervioso dorsal hueco, hendiduras faríngeas y una cola posanal. makes chordate a chordate and these developmental stages also present in horse. Horse is a mammal so it belongs to the class of Mammalia due to their mammals characteristics. So in the picture, it belongs to mammals.

that is wrong, its not thymus.