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THE GNU SONG by Flanders & Swan
Found it here

A year ago last Thursday, I was strolling in the zoo
When I met a man who thought he knew the lot
He was laying down the law about the habits of baboons
And the number of quills a porcupine has got
So I asked him "What's that creature's name?" And he answered "That's a helk!"
And I'd have gone on thinking that was true
If the animal in question hadn't put that chap to shame,
And remarked,"I ain't a helk! I'm a gnu!"

"I'm a gnu I'm a gnu
The g-nicest work of g-nature in the zoo
I'm a gnu how do you do?
You really oughtta g-know w-who's w-who
I'm a gnu spelt G - N - U
I'm not a camel or a kangaroo
So let me introduce, I'm neither man nor moose
Oh, g-no, g-no, g-no, I'm a gnu!"

I had taken furnished lodgings down at Rustington-on-sea
Whence I travelled on to Aston-under-lyne
On the second night I stayed there I was wakened from a dream
Which I'll tell you all about some other time
Among the hunting trophies on the wall above my bed
Stuffed and mounted was a face I thought I knew
A bison? An okapi? Could it be a hearty beast?
Then I seem to hear a voice "I'm a gnu!"

"I'm a gnu! a-g-nother gnu!
I wish I could g-nash my teeth at you
I'm a gnu how do you do?
You really oughtta g-know w-who's w-who
I'm a gnu spelt G - N - U
Call me bison or okapi and I'll sue
Nor am I in the least like that dreadful hearty beast
Oh, g-no, g-no, g-no, I'm a gnu!"

"G-no, g-no, g-no, I'm a gnu!
G-no, g-no, g-no, I'm a gnu!"