Jesli he/ she/ it, to chodzi o zaimek, nie rodzajnik ;-)
He/ she - mozna uzyc w odniesieniu do zwierzat domowych, ktore znasz (maja imie, itp). Podkreslasz wtedy emocjonalna wiez, np.
My dog's name is Spot. HE's a great friend and companion.
My friend's cat is lovely, although SHE can be quite aggressive sometimes.
Jesli mowa o zwierzeciu ogolnie (np. zauwazonym gdzies na ulicy) - wowczas "it"
I saw a beautiful horse yesterday. IT was standing right in the middle of the town square.