The flock of horses prefecture is the part of Japan known as Gunma Prefecture.
The readings for 群 and 県 are okay, but you haven't learned this reading for 馬 yet, so here's a mnemonic to help you:
Gunma Prefecture is where your mama (ま) lives! Imagine going back to your mama's house, and finding it's now in Gunma Prefecture. It's a bit of a surprise, but very handy for practicing your Japanese. Thanks mama!
Let me know when you come back from your parents’ house in Gunma Prefecture.