Setting up a loop to compare two columns

[EURGAW]

Hello, I realize this is a silly question but I'm very new at this so please be patient with me.

Let's say I have something like this (that I've parsed from a XML file, where the AnimalType returned 3 values in this order):

I'm trying to make it like this:

So the thing is, I want to compare the 2nd column with the first column, and if the first 3 letters are the same then it's a match! However, I have no idea how to compare these items and create an exit condition for when they no longer match. I've tried using a Do While loop but I end up creating an infinite loop that just spits out the first value.

Thanks for any help!

 

[JoeMo]

Would a formula solution like this work for you?

Excel Workbook
	A	B
1	Animal Type	Tag ID
2	Dog	Dog_1A
3	Dog	Dog_1B
4	Dog	Dog_1C
5	Cat	Cat_1A
6	Parrot	Parrot_1A
7	Parrot	Parrot_1B
Sheet1