It has been dated to at least 1927 http://www.fun-with-words.com/shortest_poem.html, as published in the Mt Rainier Nature News Notes (1 July 1927). <br class="br">Misattributed

“The garden is a raging sea,
The hurricane is snarling;
Oh, happy you and happy me!
Isn't the lightning darling?”

Ogden Nash

Many Long Years Ago (1945), A Watched Example Never Boils

“Behold the hippopotamus!
We laugh at how he looks to us,
And yet in moments dank and grim,
I wonder how we look to him.”

Ogden Nash

"The Hippopotamus" http://www.cs.rice.edu/~ssiyer/minstrels/poems/848.html

“May I join you in the doghouse, Rover?
I wish to retire till the party's over.”

Ogden Nash

Many Long Years Ago (1945), Children's Party

“Good wine needs no bush,
And perhaps products that people really want need no
hard-sell or soft-sell TV push.
Why not?
Look at pot.”

Ogden Nash

"Most Doctors Recommend or Yours For Fast Fast Fast Relief" in The Old Dog Barks Backwards (1972)

“I can't say that I feel particularly one way or the other towards bell-boy,
But I do admit that I haven't much use for the it's-just-as-well boys,
The cheery souls who drop around after every catastrophe and think they are taking the curse off
By telling you about somebody who is even worse off.
No matter how deep and dark your pit, how dank your shroud,
Their heads are heroically unbloody and unbowed.”

Ogden Nash

"Look for the Silver Lining"
Happy Days (1933)

“Man is a victim of dope
In the incurable form of hope.”

Ogden Nash

"Good-by, Old Year, You Oaf or Why Don't They Pay the Bonus?" in The Primrose Path (1935).

“A wonderful bird is a pelican,
His bill will hold more than his belican.
He can take in his beak
Food enough for a week;
But I'm damned if I see how the helican.”

Ogden Nash

"The Pelican" (1910) by Dixon Lanier Merritt is another poem often misattributed to Nash.
Misattributed

“The bear said, Isabel, glad to meet you,
How do, Isabel, now I'll eat you!
Isabel, Isabel, didn't worry.
Isabel didn't scream or scurry.”

Ogden Nash

"Adventures of Isabel" http://www.poemhunter.com/poem/adventures-of-isabel/

“When people reject a truth or an untruth it is not because it is a truth or an untruth that they reject it.
No, if it isn't in accord with their beliefs in the first place they simply say, "Nothing doing," and refuse to inspect it.”

Ogden Nash

Good Intentions (1942), Seeing Eye to Eye is Believing

“Your hair may be brushed, but your mind's untidy,
You've had about seven hours' sleep since Friday,
No wonder you feel that lost sensation;
You're sunk from a riot of relaxation.”

Ogden Nash

"We'll All Feel Better By Wednesday"
Versus (1949)

“Oh, things are frequently what they seem,
And this is wisdom's crown:
Only the game fish swims upstream,
But the sensible fish swims down.”

Ogden Nash

"When You Say That, Smile", as quoted in Saturday Evening Post, 16 September 1933

“Belinda was as brave as a barrel full of bears,
And Ink and Blink chased lions down the stairs,
Mustard was as brave as a tiger in a rage,
But Custard cried for a nice safe cage.”

Ogden Nash

Many Long Years Ago (1945), The Tale of Custard the Dragon

“Whales have calves,
Cats have kittens
Bears have Cubs,
Bats have bittens,
Swans have cygnets,
Seals have puppies,
But guppies just have little guppies.”

Ogden Nash

"The Guppy"
Versus (1949)

“Camped on a tropic riverside,
One day he missed his loving bride.
She had, the guide informed him later,
Been eaten by an alligator.
Professor Twist could not but smile.
"You mean," he said, "a crocodile."”

Ogden Nash

"The Purist"
I'm a Stranger Here Myself (1938)