K.P.C.O.F.G.S.

Zebra:

K- Animalia

P- Chordata

C- Mammalia

O- Equidae

F- Perissodactya

G- Eq

S- Caballus 

Pig:

K- Animalia

P- Chordata

C- Mammalia

O- Artiodactyla

F- Suidae

G- Sus

S- Domestica

Horse:

K- Animalia

P- Chordata

C- Mammalia

O- Perissodactyla

F- Equidae

G-Eq

S-Equidae

Cow:

K- Animalia

P- Chordata

C- Mammalia

O- Artiodactyla

F- Bovidae

G- Bus

S- Taurus

Kitten:

K- Animalia

P- Chordata

C- Mammalia

O- 

F- Felidae

G- Fells

S- F. Catus

All the animals are similar because they all share the kingdom Animalia. Also, they all share the same Phylum, Chordata. Their class is also Mammalia. The cow & pig share the same order which is Artiodactyla. All the animals have different species, genus & family. 

Question:  Why aren't the zebra & horse more similar?