D. Tomcat

The number of letters of each animal corresponds to the number of the letters in each month. And the first letters spell “Gets Goat”.

January / Giraffe = 7 letters
February / Elephant = 8 letters
March / Tiger = 5 letters
April / Snake = 5 letters
May / Gnu = 3 letters
June / Oryx = 4 letters
July / Anoa = 4 letters
August / Tomcat = 6 letters