It’s said that in this pond there is a huge leopard frog called the master of the old pond.
This has a similar meaning to 主人 (head of household), but we're mainly learning it so you can learn some other vocab later on. You can remember it also as the meaning you learned with the kanji 主, which is master.
The reading is of course different from what you learned with the kanji. Think of this: The person who was your head of household was very strict. Anytime you did anything that was incorrect or bad, he/she would pull out the noose (ぬし), but would call it a "noosey" to be cute, since you were just a kid.