Americans love nicknames. Even when there's a nickname they will use on their kid forever, they will pick a longer name, even if it means making one up. For example, when the name Brandi started becoming popular for girls, the longer forms Brandisha, Brandina, Brandolyn, and Brandice were coined. For boys, Jax has become popular. Not satisfied with Jackson as a long form, parents have coined Jaxton and Jaxion. There are countless examples in American naming history of this, backforming long versions of names (or using already existing names and places in a new context) to get a nickname.
This phenomenon is more common for girls, but there are some examples for boys as well.
here are some more examples:
Kelly -> Kellina
Lexi -> Lexington, Alexia
Kaylee -> Kayleena, Jakaylee, Kaylisha, Kaylissa, Kaylise
Jody -> Jodene, Jodette, Jodelle
Grace/Gracie -> Graceland, Graycen
And for boys:
Cody -> Dakota, Koden, Jacody
Trent -> Trenton, Trentavious
Gray -> Grayton, Graylan
Bodhi -> Boden, Bowden
Brad -> Bradrick, Bradney, Bradson, Braddon, Braddox
Which brings me to the name Finn. Finn is one of those names that suddenly came out of nowhere and is now the Next Big Thing. Actually, it probably has surpassed the "next" phase and IS the Big Thing. With the rise of Finn are a bunch of Finn names that could beget Finn as a nickname:
Finnick (thanks, Hunger Games!)
and to a lesser extent:
Why stop there? I've come up with some more you could use:
Infinity -it's been quietly used for both girls and boys for a couple of decades now!
Delphinus -it means "from Delphi" and is the name of a saint who died in the year 403.
Arnfinn -it means "eagle of the Finns" in old Norse
Fingal -it comes from Fionnghall, meaning "white foreigner" in Scottish Gaelic
Rufinus - a nickname for a red-haired person in Ancient Rome
Finesse -it's been used sporadically as a name since the 90s. Why not? Abstract concept names are all the rage right now!
Serafin/Seraphinus - meaning "fiery ones," the Seraphim are an order of angels in the bible.
Finis - it means "the end" in Latin. Make of that what you will.
Griffin - a mythical beast. Why not?
Phinizy - this name was given to 5 boys in 1920 and hasn't been used since. Bonus: it's the name of a swamp in Georgia!