Cinnamon is Raj's dog, a Yorkshire Terrier given to him by Howard and Bernadette in "The Transporter Malfunction" (S05E20) after his failed date with Lakshmi, so that he could get through his loneliness without having a girlfriend. Raj shamelessly pampers her, cooking her elaborate meals and taking her for walks in a pet stroller, which prompted Howard to remark to her that he "has a crazy mother, too."
Cinnamon's first appearance was when Bernadette pulls her out of her purse to give to Raj. Her fur is shorter and she is smaller being only probably a few months old. He immediately coos and hugs the pup, calling her the "cutest little Yorkie I've ever seen. He at first assumes she is a he but Bernadette and Howard correct him saying the two of them would hit it off. Raj thanks them and then tells the pup they'll go see if she'll fit in his man purse. This causes Bernadette to mutter to Howard "Heterosexual, my ass."
In "The Proton Resurgence" (S06E22), Howard and Bernadette looked after Cinnamon while Raj worked a long weekend in the telescope lab, but they accidentally lost her during a trip to the park. When she was found and returned to him, Raj berated a worried-to-death Howard and Bernadette over Skype for losing her, only to have Bernadette berate him when he revealed that he didn't call them right away to let them know that Cinnamon was safe and sound.
In the beginnng of the episode, Raj asks Howard to watch over Cinnamon while he is working in a telescope lab. Howard asks why not put her in a kennel and he gets defensive asking why Howard doesn't put his mother in a home. Howard claims his mother would be better in a kennel.
Raj comes in with a pink baby stroller and reveals his precious pup to "Uncle Howard". (There are oos and awws from the audience at her reveal.) He says that Cinnamon is having a sleepover party with her "Uncle Howard" and "Aunt Bernadette". Howard makes a stale comment that Cinnamon would be adorable until Raj had a stroke she'd eat him. This does not phase Raj as he claims he would be happy to be her "nom-noms". Raj hands a backpack to Howard lists all the things Howard will need to care for Cinnamon. In terms of feeding, she'll eat fancy veal or scampi for dinner and egg fritta with choice of homefires or English muffins for breakfast, but not both because both of of them are "watching their weight". Howard calls her a "classy dog". Raj then tells him not to leave the lid on the toilet seat up as she will drink out of it, proving that Cinnamon is still a dog no matter how much she is pampered. Howard says he feels for saying that he's got a "psychotic mommy too". (Notice that Cinnamon is wearing a pink collar and a barette in her fur).
Howard and Bernadette take the pup to the park and come back with her pink stroller. Bernadette comments on Howard's throwing abilities and claims he was being considerate in making sure Cinnamon didn't wear out her legs. Bernadette also comments on how the three of them made her feel like a family. This brightens Howard's hopes of one day having a family, especially if they are as quiet as Cinnamon. Howard lifts the hood of the stroller to reveal that she is gone. Panic ensues between the couple as they blame each other for not watching over her and Bernadette says that Howard throws like a girl.
Howard and Bernadette go back to the park in search of the lost pup. They call out for her but get no response. Bernadette suggests that she can't understand their calls because of Raj's accent. Each tries to imitate her master but still get no response.
Her master gets a call from someone saying that he found her in the park. Raj picks her up and takes the shaken pup for massages. (The question is if Cinnamon was really shaking in fear or that was her natural state of movement.) Raj gets in touch with Howard and Bernadette as Howard was printing out "lost dog" posters with Googled images of "foo-foo little dogs". Howard claims that Bernadette took Cinnamon for a walk until Raj calls him a liar and reveals the pup on webcam.
Lucy, Raj's neurotic not-quite-yet girlfriend, met Cinnamon in "The Closure Alternative" (S06E21) and thought that she was really cute, despite Raj's claim (as he was trying at the time to convince her how "manly" he was) that she would "rip her face off with the right command."
Cinnamon re-appears in Season 7 in "The Discovery Dissipation" (S07E10) when Raj is staying with Howard and Bernadette. She and her master are in need of a home as their apartment is being renovated and there is no hotel that will take pets. Seen in her stroller at the beginning of the episode with Raj is bringing in lattes and a hot chocolate for his hosts she vanishes from shot until Raj lifts her up and allows her to lick his face. Howard comments on how she uses her tongue to clean her butt. Raj tells him that's why he brushes her teeth because the other half of her tongue time is licking Raj's face.
She appears at the end when Raj has been kicked out of Howard and Bernadette's apartment for being the ideal house guest and causing the couple to fight. She is seen moving around in her carrier as Leonard and Penny make up the couch for Raj. Raj claims he left her toothbrush at Howard and Bernadette's and says she'll be sharing with "daddy". Disgusted, Penny tells her to bark once if she wants Penny to call the PETA when Sheldon and Amy walk in. Sheldon immediately freaks out and Leonard tries to re-assure Sheldon that she's in her carrier and can't get out. Sheldon immediately compares the cute little dog to Juarassic Park's dinosaurs and how if they escaped so could she. What happens to her after Raj gives Sheldon and Amy relationship advice, is unknown of whether they were kicked out or they stayed.
Cinnamon comes back into the series in "The Locomotive Manipulation" (S07E15) as Raj drops her off with Leonard and Penny while he is in the telescope lab again. His first choice was probably Bernadette and Howard but they were on a train trip with Sheldon and Amy. Cinnamon eats Leonard's Valentine's chocolates and ends up at the vet. Raj is very upset at Penny and Leonard, but his devotion to his dog touches the vet who slips her phone number into her instructions.