What is Cookie Monster's (Sesame Street) name?

His name is Sid.

https://www.smithsonianmag.com/smart-news/cookie-monsters-first-name-sid-and-other-icons-real-names-180950099/

There are several answers to this question: The Wheel Stealer, Arnold, Sid (aka Sidney Monster) and Alistair Cookie.

In 1966, Jim Henson created three cookie eating monsters supposed to appear in a snack food commercial: The Wheel Stealer, the Flute Snatcher and the Crown Grabber. The commercial was made but never aired. Here it is on Youtube:

https://www.youtube.com/watch?v=5QngRWeEJGo

The Wheel Stealer was later seen as a munching monster in a Munchos commercial:

https://www.youtube.com/watch?v=xF7BlsSzHVQ

All three monster became recurring characters on Sesame Street: The Crown Grabber was later known as the Beautiful Day Monster, the Flute Snatcher turned into Snake Frackle. The Wheel Stealer, however, made his next career step around 1968 as Arnold, the Munching Monster in an IBM commercial

https://www.youtube.com/watch?v=7IgF6_jVaj8

before becoming the Cookie Monster on Sesame Street. Frank Oz (the monster's main puppeteer) later revealed that his name was Sid Monster.

http://news.bbc.co.uk/2/hi/entertainment/4432415.stm

By the way, on Sesame Street's official Twitter account, the monster calls himself Sidney.

https://twitter.com/sesamestreet/status/20796273130

Last but not least: When hosting the Monsterpiece Theater his name was Alistair Cookie:

https://www.youtube.com/watch?v=pudBqiTKA7U

Cookie Monster's real name is Sid! Who knew? He apparently revealed his real name in a song in 2004 and then later confirmed his name in 2017.

https://en.wikipedia.org/wiki/Cookie_Monster