Thor and the rest of the Asgardians (or at least the guards in Endgame) are always referring to Rocket as a rabbit in both Infinity War and Endgame. Out of universe, the explanation would be that its funny, but I was wondering if there's an in-universe reason. Evidence from the MCU is preferred but you can draw from comics etc that aren't part of the MCU.
2 Answers
The film's (canon) prose novels offer us a little more info. Thor calls him a rabbit because he thinks Rocket is a rabbit. While this doesn't directly address whether rabbits exist on Asgard (or whether he's seen one during his brief time on Earth) it at least shows that he's not being facetious.
"Is it weird that I wanna do it even more now?" Rocket asked.
"Mmm, a little bit. Yeah," Thor answered, questioning his choice to bring the rabbit along.
Rocket accepts that it's just because he's simply ignorant.
“You’re in a foul mood, rabbit,” Thor said, shaking his head.
“I’m in a foul mood? Yeah, I wonder why,” Rocket said sarcastically. Surely it had nothing to do with pirate-angel progeny constantly mistaking him for a rabbit.Avengers: Endgame the Pirate Angel, the Talking Tree, and Captain Rabbit
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9It might also be worth pointing out that Thor is a Norse god, and there aren't any raccoons native to Europe. It's entirely possible that he simply didn't feel like there was any pressing need to acquaint himself with the wildlife of the Americas. Jul 7, 2019 at 13:35
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But there are raccoons in New Mexico, so there likely are some in "Ancient Bridge", or Puente Antiguo. I think at least part of the explanation is that Thor isn't super observant. I'll bet Hemdal wouldn't have made that mistake.– userLTKOct 21, 2019 at 13:07
In this clip from Guardians of the Galaxy, Rocket didn't know what a raccoon was either, and denied that there was any creature like himself.
Rocket travels a lot, so this suggests that there are lots of planets without raccoons. For all we know, Earth could have all the raccoons in the universe.
Rocket is kind of an idiot though, so there is also a possibility he is mistaken. He was created by cybernetically enhancing an animal; presumably this happened near a planet that has raccoons or raccoonoids.
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7I think narcissism over idiocy is far more likely. Rocket believes he's pretty cool.– user15742Jul 7, 2019 at 7:16
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5In the first movie, Drax at least thinks he's seen creatures like Rocket before, despite never having been to Earth: "I recognize this animal. We'd roast them over a flame pit as children. Their flesh was quite delicious." Jul 8, 2019 at 1:12