The genitive (possessive) case is made by adding an s to the singular form of the noun, e.g. the doctor's surgery.
With most plural nouns, like lasoos, an s ending is already present, so the the genitive case (whose main meaning is possession) adds a following apostrophe rather than another s.
So, a typical use of the plural possessive of lasso would be "the lasoos' knots were defective".
Hope that helps.