- "You must feel the Force around you, here, between you, me, the tree, the rock, everywhere. Oh, except within ten meters of an ysalamir."
"Uh, lizards... block the Force they can. Hastily along we move..."
"Wait, wait, are you telling me all my training is useless if there's a lizard in the room?"
- ―Yoda, teaching Luke Skywalker the weird Expanded Universe rules of the Force
The ysalamir, plural ysalamiri, was a lizard invented by Timothy Zahn that had the ability to neutralize the Force. It seemed a harmless enough thing when Zahn invented it, because it gave Grand Admiral Thrawn the ability to look down his nose and sneer at any Jedi or Sith who thought they could mess around with him. But it opened up a whole big can of worms, like, wait, how...? Why...? And if that's true, then... Uh-oh...
Ysalamiri evolved on the planet of Myrkr, which was an out-of-the-way planet that no one visited... but that was on purpose. Actually, Jedi and Sith kept rediscovering the planet, finding that there were animals there that could block the Force, and then they'd desperately try to keep people away. A critter like that could ruin the sweet gig they had, ruling the galaxy and all. Apparently, the ysalamiri were the evolutionary reaction to the first Force-using creature, a predator called a vornskr. Vornskrs used the Force to hunt, which is actually pretty cool. In point of fact, some of the more, er, rural Jedi also used the Force to hunt... they'd pack up their repulsor-pickup-truck with plaid shirts, shotguns, and beer, and shout "yee-haw" a lot. The ecosystem of Myrkr was temporarily devastated by packs of vornskrs, who became the dominant predator on the planet and drove many species to extinction. But then the hapless prey happened to evolve the ability to block the Force, so the vornskrs had a tougher time killing them. Um. Or maybe it was the reverse. Maybe the ysalamiri evolved first and... wait, no, that doesn't make as much sense.
Ysalamiri were annoying little creatures. For one thing, they spent nearly their whole lives clinging to a tree branch. They dug their claws in and fed on the sap. And they pretty much never left, unless it was absolutely vital to their survival. They couldn't be removed from their precious tree branches without killing them. So when Thrawn, or Tyber Zann, or Talon Karrde, or Admiral Daala, or whoever, tried to harvest ysalamiri to keep themselves safe from Jedi, they basically had to take the whole tree. Thrawn came up with a rather stylish way of handling it, by inventing a kind of shawl thing with a nutrient frame. Then he'd wear the ysalamir like a mink stole. It looked a little gay, but it was worth it for the bad-ass moment when some Sith Lord would try killing him with Force lightning.