On Twitter, Mr. Clean isn't Procter & Gamble's bald-headed muscleman. He's a designer from Pennsylvania. As for Mr. Peanut, there are two: a French-Canadian and a nut that wields an ax.

Many of the most-loved brand mascots, from the Pillsbury Doughboy to Tony the Tiger, are on Twitter in some respects, but not in any official capacity. Everyday people not affiliated with the brands have picked up those familiar names and cartoon faces, as many marketers have left them idle. @TheChefBoyardee, for example, is foul-mouthed, wears a chef's hat and applied to Charlie Sheen's #tigerblood internship.

"Brands can't do anything about [fake Twitter handles] other than complain or set up a verified account, but they need to put something into its place," said SapientNitro Creative Director Derek Fridman, who hosted a panel on this topic at the South by Southwest Interactive conference last month. During the panel, a real McCoy, @Smokey_Bear, responded to people tweeting with the hashtag #brandmascots. "People often want to see those brands online and will create them on their own and assume the role."

One reason why so few are online might be because many brand mascots are highly controlled. Long, detailed marketing documents often dictate exactly what these cartoons can wear, say and do. But leaving these highly protected characters voiceless in social media has opened the door for the public to drag these highly guarded brand characters into dicey territory.

If people aren't making money off brands' character copyright, marketers aren't likely to be able to claim legal damages for misrepresenting, say, the California Raisins. Yet, "even if they are used for non-commercial purposes, I think it would be prudent for brands to be vigilant in protecting their assets because consumers might well believe there's some connection here," said Linda Goldstein, chair of advertising division for law firm Manatt, Phelps & Phillips.

Twitter does not police user accounts, but does accept reports from individuals or companies about accounts that want to take down impersonators. Some brands have contacted Twitter to reclaim accounts that use their trademarks or characters and, increasingly, those requests come through the social network's growing ad sales force. But Twitter has strict rules about these accounts: Parody, fan or commentary tweeters are just fine. It is accounts with clear intent to deceive or confuse that are prohibited as impersonation accounts and subject to suspension.

With famed mascots such as the Pillsbury Doughboy and Tony the Tiger unclaimed on Twitter, regular people are picking up those brands and tweeting what they wish. One account, @PillsburyDboy, uses images of the General Mills mascot for its profile picture and background. His name is listed as "Pillsbury Doughboy" and his location is grocery stores. "Being the pastry mascot, my life is routine -- get bought at a grocery store and eaten later on everyday," he once tweeted. After a period of frequent tweeting, Dboy appears to have abandoned the account in October 2009.

As for Mr. Clean, if P&G ultimately decides to push the more than 50-year-old man onto Twitter, it plans to ask that his eponymous handle be returned to the brand, said a spokesman. But nothing's happened yet. "Mr. Clean does not have an official Twitter handle, and no steps have been taken to reclaim the brand name from the fan who created the account," said a P&G spokesman.

Mr. Peanut, who only uttered his first words months ago after being silent since 1916, is not ready to debut on Twitter, said a spokesman for Kraft, Planters' parent company. He is, however, already very active on Facebook and Kraft owns the @MrPlanter handle. As for squatters such as @MonsieurPeanut and @Mister_Peanut: "We have no plans to share at this point regarding other Twitter handles that are similar to Mr. Peanut's," the spokesman said.

Not all old mascots are missing on Twitter. General Mills operates an account for Green Giant, the massive mascot for its frozen-vegetable products. While @GreenGiant has been uttering wholesome tweets like "Do you have any fun weekend plans? Lil' Sprout is hanging out in Costa Rica!" since October, a Washington pot dispensary has been spreading the marijuana gospel from @420jollygreen, using General Mills' Jolly Green Giant name and image. General Mills did not provide comment on this account.

"This is something brands should want to police," said Manatt's Ms. Goldstein. "This kind of activity could result in tarnishing of the brand."

One newer mascot has taken action to dethrone squatters. Aflac, with help from digital agency Digitas, has run @AflacDuck for its famous 11-year-old mascot since early 2009. After Ad Age inquired about a copycat account, @RealAflacDuck, the brand's new media director James Wisdom said it was in the processing of having the account shut down. The account has since been suspended.

Quaker Oats' cereal brand Cap'n Crunch also recently pushed its mascot on Twitter. The move coincided with a Chicago boutique social-media agency's guerrilla campaign to cajole the PepsiCo character onto Twitter. The agency Giant Steps launched a "Where's the Cap'n?" website, Twitter feed and Facebook page and handed out missing posters. The agency wasn't responsible for getting the brand onto Twitter, said Barbara Liss, Quaker Oats director of social media and digital, but it definitely caught her attention. Last month, Quaker launched @RealCapnCrunch and has so far seen limited returns.

For one, it's been a challenge to update the decades-old brand for a 21st-century medium. "We're working to figure out how do to use things that are relevant in pop culture and also makes sense for the Cap'n," Ms. Liss said. For now, that mostly means thanking those who are tweeting about the cereal. After about one month, Ms. Liss said she hopes to see more followers than the Cap'n's current 1,500.

Not all brand mascots can weather current events. Shamu, Sea World's beloved killer whale, resigned Twitter after the real-life whale killed a trainer. "At this difficult time, @Shamu will not be active," he tweeted for the last time after the accident. "For Twitter updates follow @SeaWorld_Parks."

Other recent brand characters have eschewed Twitter on purpose. Heineken brand Dos Equis decided against taking its Most Interesting Man into the tweetosphere.

"When we evaluated if Twitter should be part of our marketing mix, we felt that we also needed to apply filters to stay true to our brand character," said Paul Smailes, senior brand director for Dos Equis and Sol beer brands. The man has also turned down movie studios and other advertisers looking to partner. "We believe the Most Interesting Man doesn't spend too much time with technology. He's busy with his worldly adventures."

After decades on packages, in print and on TV, Tony, Mr. Clean and Doughboy could probably make the same claim.