In America, langostino refers to meat from the squat lobster. This can cause some confusion, because in the UK it means scampi (legally defined specifically as the Norway lobster, sp. Nephrops norvegicus). Meanwhile, in Spanish it usually refers to prawns, although in Argentina it refers specifically to the Argentine red shrimp, and in parts of the Caribbean it can mean a crayfish.
In the United States the Food and Drug Administration only allows for langostino to be used to market three species of squat lobster: Cervimunida johni (sometimes called the blue squat lobster), Munida gregaria (the gregarious or swarming squat lobster), and Pleuroncodes monodon (red squat lobster), all of the family Galatheidae.
To be absolutely clear, fishermen catch small, lobster-like things and sell them at market without any genetic testing whatsoever. However...
There have been some minor scandals when seafood restaurants are caught selling langostino labeled as lobster. Squat lobsters are closely related to porcelain crabs, more distantly related to hermit crabs, and, if you go up the tree far enough, eventually, lobsters. So this is clearly false advertising. A number of American resturants have been caught doing this, including Long John Silver's selling 'Lobster bites' that contained no lobster (2006), Rubio's Restaurants selling "lobster" burritos and tacos (2006), and Red Lobster selling "lobster" bisque (2016). While these all caused minor kerfuffles, in each case the problem was solved not by pulling the langostino dishes from the menu, but by correcting the labeling and advertising; consumers don't seem bothered.