How can I add a dictionary variable or lookup-table variable in dataset

I have a very trivial question, but I wasn't able to solve it so far-

in my dataset I have a 'cowID' number which goes in a non-linear way from 2 to 9130. every cow has a tag. let's say cow 2 has tag 1, cow 230 has tag 2, cow 9130 has tag 3 etc. I want to add a 'TagID' variable which runs all the tag numbers corresponding to the cow numbers. THAT'S IT!

tnx in advance