Combine 2 columns into 1 column, order is in a "zigzag" fashion

Hi,

I'm looking to combine two columns into one, but the order is dependent on the value in the first column. As the value in the first column repeats, it will stack its values in the second column below it until it finds a new unique value in the first column, then it does the same for this new unique value. Not sure if a lookup/match function could work. Any ideas? See sample workbook attached:

Pasadena.xlsx

Example:

A ant
A bird
A cat
B dog
B elephant

Result:

A
ant
bird
cat
B
dog
elephant