I remember that Hermione was nicknamed Hermy by someone in the HP series. I am not sure of the answer, so I have asked it out.
Hagrid did, in order to make it easier for his big little brother to remember her rather more difficult full name:
“An’ this is Hermione, see? Her—” Hagrid hesitated. Turning to Hermione, he said, “Would yeh mind if he called yeh Hermy, Hermione? On’y it’s a difficult name fer him ter remember.”
Order of the Phoenix, Ch. 30 - Grawp
It was Grawp, Hagrid's half-brother. He couldn't pronounce Hermione: Hermy was the best he could do, best seen when Umbridge, Harry and Hermione are in the Forbidden Forest togther:
'"HERMY!" roared Grawp. "WHERE HAGGER?"'
- The Order of the Phoenix, chapter 33, Fight and Flight, p. 758
He couldn't manage Hagrid, either (he went with Hagger).
EDIT: Krum also had trouble with her name, and pronounced it "Herm-own-ninny", though Grawp is most likely who you're thinking of (Krum only said it wrong once, and it wasn't a nickname).