There are a variety of ways to form casual address terms.
Using family terms is common ('brother'), shortening/modifying a given name ('Teddy' from Theodore), or something based on the person's characteristics ('Shorty').
What idea do we have of what people did in the past?
I know a lot of our sources are official, and less likely to have informal names. But what info DO we have?

PS. General trends for a culture, or specific examples, are both appreciated.
PSS. I'm most interested in the Early Medieval/Post-Classic period - 500AD-1200ADish. And/or in the Americas, India, SE Asia, Eastern Africa, Japan, Ireland, and the Middle East. But any and all anecdotes/facts would be appreciated. Onomastics is fascinating.