import math
word_list = ['Cat','Dog','Bird','Goose','Llama']
with open("/Users/andrewlintz/Desktop/findreplaceprocess/doc2.txt") as main, open('/Users/andrewlintz/Desktop/findreplaceprocess/doc3.txt', 'w') as new_main:
    counter = 0
    for line in main:
        temp = line
        num = int(math.floor(counter/8))
        counter += 1
        temp = temp.replace("MARKER",word_list[num])
        new_main.write(temp)