What's in a name?

[Guu]

The TV tropes page gives sort of an explanation where Guu's name comes from:

The title of the series is a rather elaborate pun that can be read several different ways, due to different readings of some of the words:
In the Jungle was Always Hare but then came Guu
The Jungle was Always Nice, Then Came Guu
The Jungle Is Always Sunny or Hungry

and the most obscure, a pun on a common phrase in Japanese weather forecasts:
The Jungle Is Always Clear, With A Chance of Showers
The Jungle is Always Clear, With Scattered Guu
(where "Hare" means "clear" and "Guu" means "showers").
Any way you read it, it means Guu is turning Hare's life upside down.

As for Lopunny, I believe her name is a combination of "Bunny" and "Lop." As in big loppy ears.