Answer :

There are multiple solutions. One is 7 chicken and 1 cow.
Albina
1/ 7 chickens and 1 cow >>> 7 x 2 + 1 x 4 = 18 (legs)

2/ 5 chickens and 2 cows >>> 5 x 2 + 2 x 4 = 18 (legs)

3/ 3 chickens and 3 cows >>> 3 x 2 + 3 x 4 = 18 (legs)

4/ 1 chicken and 4 cows >>> 1 x 2 + 4 x 4 = 18 (legs)