Locoweed is not one specific plant, but it's one of a number of species that produce a toxin called swainsonine. It's also called crazyweed.
As you would expect from the name - locoweed poisoning makes horses (and other livestock)...crazy. Specifically, it makes them depressed and withdrawn, although some animals may become anxious or even violent. Locoweed also causes malnutrition because the animals become too lethargic to bother to eat and often lose their sense of direction, and it can cause irregular gait, loss of libido, and infertility - including miscarriages. Birth defects are often common.
Some animals recover, but permanent neurological damage is not uncommon and horses that are "loco" are often no longer useful as working animals - and may be sterile. Also, for some reason, an animal that's sampled locoweed once tends to go back to it. The stuff is apparently tasty. There's no treatment for it, either - the only solution is to keep animals away from it, especially if there's no other good forage. It is possible to spray with modern herbicides.