buses option from kowloon to guangzhou zoo

There is no direct bus from Nathan Road to Guangzhou zoo, and you are suggested to take a train not a bus.
You can take the MTR to Hung Hom Station, where you can take the train to Guangzhou East. From 07:25 to 20:01, there are 12 trains available. The travel time is about 2 hours, and the fare is HKD190.

Upon arrival in east railway station, you can take the subway line 3 to Yantang Station, where you can transfer to line 6 to Shaheding Station. Take the Exit A and walk about 430 yards, you will reach the zoo.

By bus: Walk to Nanking Road (near exit B2 of Jordan MTR Station) to buy a ticket in the office of Chinalink Bus Company and take a bus to Guangzhou. It has several drop-off stations and I suggest you get off at Guangzhou Hotel (near Haizhu Square). Then take a taxi to the zoo in 10-15min.