Two bulls in a living room. One bull is sitting in a chair, blowing up an inflatable cow. The other is standing beside him, rubbing his hooves together in anticipation and saying, "She’s looking reeeeeeal good, Vern!!
Suddenly, in the middle of the flock, the cook is goosed!
and
When animal mimicry goes bad.
A horse has skidded to a stop right at the edge of a gorge. The rider has been thrown off and is plummeting in. The horse thinks: “Shoot. I’m gonna go back and try this again.”
A group of penguins and a polar bear wearing a simple penguin mask are on a very small floating chunk of ice. The caption is something like: “And now Floyd’s missing, too. I tell you, something funny’s going on.” This one is my absolute favorite just because of the way the bear is drawn.
Looking down from a castle wall as viking invaders are storming it, the top of the simple wooden ladder says “This is not a step.”
The Donner Party memorial is probably the funniest one.
I second loving “Custer’s Last View.”
Two of my favorites that haven’t been mentioned yet. One is “Washington Crossing The Street” and the other one has two ancient Chinese warriors standing atop the Great Wall of China, and one warrior says to the other one, “Now let’s see that dog get in here!”
Title: Predator-Prey relations
Cartoon: A wolf and a sheep meet on a moonlit hilltop. The wolf is giving the sheep a bouquet of flowers.
Caption (paraphrased): Eventually their relationship ended. Bernie worried about what the pack would think, and Sheila just ate the flowers.
Dogs are in a classroom. One dog is pointing at a projection of a cat with its fur raised. “As you see, the cat will puff up its fur so as to appear bigger. Deception, deception, deception!”
Seconding “Cat Fud” and the goldfish dinner party (seconds).
So I’ll add:
Dog standing outside his suburban home, sucking on a garden hose. Doggie wife (beehive hairdo, glasses) standing on the porch, says disgustedly, “So…planning on spending the day roaming around the neighborhood with your friends?”
So many of the ones I love have been mentioned. “Midvale School For the Gifted” will always be my absolute favorite (because I do that all the time), and “The Holsteins Visit the Grand Canyon,” which is funny enough on its own, and only gets funnier when you read Larson’s little blurb explaining that the one cow is not wearing a bow on her head, it’s the other young cow doing the “bunny ears”. Probably because almost every picture of my brother and I taken between 1980 and 1985 shows me with finger bunny ears.
Also, I could swear the one with “Trouble brewing” as the caption showed a day-care center next to a dingo farm.
Another that never fails to make me giggle is “Ship of Fools/Car of Idiots.” There’s no describing it–it’s exactly how it sounds.
There’s also the woman driving in her car, looking terrified and thinking “It’s okay, there’s just a bee in the car, all I have to do is open the window and let him fly out …” In the backseat, a man-sized bee.
Ah … I love irony. 
Mating calls of various species:
Frog: Ribbit. Ribbit.
Crow: Caw, caw!
Dog: Woof, woof!
Human: Hey bay-bee, hey bay-bee.
So many great Gary Larson strips. Hard to believe that so many classics could come from one person’s mind.
My three favorites:
-
Two guys in “space suits” in a bio research lab. They have clearly been arguing - one says to the other, “Oh, so that’s the way it’s going to be, huh?” as he rips a big hole in the guy’s suit.
-
Room full of scientists and mathematicians. One of them has a sign pinned to his back, but instead of “kick me” it has a bunch of equations. The other mathematicians are pointing at it and giggling at each other behind his back.
-
“Rocket scientist” type is working on a nuclear missile. He has the inspection panel open and is fiddling around inside with a screwdriver. Meanwhile, his colleague has sneaked up behind him and has an inflated paper bag that he is about to burst.
There were several “Trouble brewing” strips. The aforementioned poodle/falcon and daycare/dingo, as well as one that showed a skydiving school right next to an alligator farm.
This thread has really brought a smile to my face. I love his plays on cliches. Like the Butler convention with hundreds of butlers standing around, one lying on the floor with a knife in his back. The detective is saying, “I hate Mondays.”
And the one with a goldfish bowl, the water almost down to the bottom, a very nervous goldfish looking up at a kid with a straw in the bowl saying, “Still won’t talk, eh?”
And the classic “Imbeciles of America” march where the marchers are holding the sign upside down. 
“Now go to sleep, Kevin—or once again I’ll have to knock three times and summon the Floating Head of Death.”
“Yes, Mr. Frischberg, I thought you’d come…but which one of us is the real duck, Mr. Frischberg, and not just an illusion?”
“And down here we keep Fifi…we’re afraid she’s gone insane.”
An elderly Superman is standing by a window, ready to leap out. He has his head turned towards an elderly woman and says “Dang! Now where was I going?”
Another one in the “It’s funny because it’s true” department:
Scene: A classroom full of students, one with his hand raised.
Caption: “Mr. Osborne, may I be excused? My brain is full.”
That’s my favorite, too. But the actual caption is better, I think: “Well, well – another blond hair…Conducting a little more ‘research’ with that Jane Goodall tramp?” Personally I like to think that last word is suddenly shouted.
Her then-executive director had a lawyer send a letter of complaint to Larson’s company. Since Jane loved the cartoon as you said, she got angry at the director and meant to send a letter of apology, but didn’t get around to fixing the matter until National Geographic wanted to reprint the cartoon.
That’s where it’s from! I took a 5-week crash course at the beginning of the summer, and it was a lot of information in a very short time. Many days, I’d be driving home, after being in class for 8 hours, knowing I had a couple hours of reading and studying ahead of me, and I had this voice in my head saying, “I need to be excused. My brain is full.”
I couldn’t remember where it was from.