Wrinklers

Wrinklers are twitchy leech-like animals that reside on the Big Cookie. Each wrinkler withers 5% of your CpS as well as storing the total amount of withered cookies, and gives back 110% of what they have stored when you pop them by quickly clicking thrice. In essence, they reduce your passive cookie production but give back much more if you let them be. They appear with the first stage of Grandmapocalypse, and the spawn rate increases with each stage.

Overview
While wrinklers reduce your CpS, this only affects the passive cookie production. Everything else, such as clicking, Luckies, Ruins, CpS based costs, and plants which give an amount of cookies based on CpS when harvested are not affected by wrinklers as they use unwithered CpS for their calculations.

You can have a maximum of 10 wrinklers eating, but the Heavenly Upgrade "Elder Spice" increases the cap to 12. When 10 regular wrinklers are eating simultaneously without any wrinkler related upgrades or Skruuia it multiplies how much you would get from passive production by 6x. In the same scenario except with 12 regular wrinklers you would get 8.32x, making "Elder Spice" incrase gains from wrinklers by roughly 39%. This spreadsheet shows the gains from differing amounts of relevant upgrades, regular wrinklers and shiny wrinklers.

During Easter season wrinklers can drop Easter eggs, and similarly during Halloween season they can drop Halloween cookies. They also change appearance during Christmas season, but this is purely cosmetic. One of the "rare" Easter eggs named "Wrinklerspawn" gives an additional 5% bonus to cookies gained when popping a wrinkler, and so does the Heavenly Upgrade "Sacrilegious Corruption".

There is a 0.01% chance for a Shiny Wrinkler to spawn, and popping one gives 3x as many cookies compared to normal wrinklers as well as the Shadow Achievement "Last Chance to See". A shiny wrinkler requires you to click 12 consecutive times in order to pop.