Frank II’s full name was Franklin Harold Longbottom.
After the second wizard war, it became common for parents to name their children after the war heroes, and names like Hermione, Ron, Neville and even Luna became popular. But of course the name of the chosen one was the most popular and it was not uncommon to see a young wizard barring the name of “Harry.” But the first in line was Harry’s loyal friend, Neville Longbottom.
"I know it’s only the middle name," Neville said, wringing his hands as he presented the idea to his friend and hero, with his seven-month pregnant wife, Hannah, standing beside him, "and I know that your full name is Harry and not Harold, but we were thinking Harry didn’t sound quite right as a middle name, see? But we still wanted to name him after you because you’ve done so much for us, Harry, so much for everyone. And I’d understand if you didn’t want us to…" Neville looked up to see tears forming behind the round glasses.
Harry embraced him and said, “I could never be more honored, Neville.”
[This is my response to an ask I received about Frank Longbottom II’s middle name]