Pinkie Octo Pie
Not just eight, thousands. They're all about the width of a hair, so they just clump together to complete her disguise. When Pinkie had her breakdown in Party of One, she had to focus more on making her imaginary friends than keeping her disguise, so the tentacles just went limp, giving the droopy hair look. That also accounts for the color change, as she couldn't keep the chromatophores their unnatural bright pink and instead dimmed to the more natural gray.
This is all part of the "pinkie pie is a silicon-based starfish alien" theory that i just made up now on the spot.