Most minor Yo-kai in the anime appear either as one-off gags or for adding to visualization or plot purposes, and do not appear in the manga or video games. The Yo-kai that appear in the anime are listed by chronological order on which they appear in.
List of Anime-only Yo-kai
Yo-kai Watch anime series
Golem Wicked
Golem Wicked (Japanese: ??? ???) is a Yo-kai of the Wicked Tribe that only appears in Yo-kai Watch: The Movie.
He is summoned when Dame Dedtime farts in front of Nate and the others.
Gnomey Army
A group of Gnomey appear in Gnumerous Gnomey, that attempt to stop the teacher from handing out a tough test at school.
Summoning Singers
When Yo-kai are summoned in the Yo-kai Watch Model U, there are several unnamed characters that accompany the summoning Yo-kai which only make an appearance in the anime series. Their appearance varies on the Tribe, such as them being rockers associated with Shady Yo-kai and hula dancers with Heartful Yo-kai.
Some of them reappear in Yo-kai Watch!, where they wish the contender good luck on fighting the opponent at the tournament.
John
In EP084, Robonyan F shows Yo-kai Scritchy (whose name is revealed to be Sarah) the future of the world if he doesn't lay eggs and give birth to mosquitoes to stop them from going extinct, and tells them about John.
Lazy Sundae's Office Workers
They appear in Yo-kai Lazy Sundae.
Blazion's Former Girlfriends
After Flushback inspirited Blazion in Yo-kai Flushback, he suddenly remembers about his former girlfriends that dumped him, causing the Yo-kai to lose focus and think too much about his past experiences.
Imagining Red and White Song Battle Competitors
Several Yo-kai appear in Whisper's fantasy in EP091 that are of participants of the Yo-kai Red and White Song Battle. They consist of a mint-green and white squirrel Yo-kai wearing a red bow-tie, a brown cat Yo-kai with yellow ears and a white face marking wearing a long-sleeved shirt and a tie and a light-blue komainu with yellow tiny hitodama flames, circular eyes blue tiny eyebrows and a green bow-tie holding a lollipop.
Song Medal Announcer
The Announcer (Called AD Yo-kai (Japanese: AD妖怪 e di yokai) in the Japanese credits) appears in EP091 and Yo-kai's Favorite Hit Music Station! that narrates the intro of the Song Medals before the singing starts.
Whisshogun
Whisshogun (Japanese: ブシパー Bushipā) is the Fusion of Shogunyan and Whisper of the Eerie tribe that appears in “Shogunyan Wants to Cut Hard Things”.
When Shogunyan went on a rampage due to his obsession with wanting to cut tough material, Whisper and Jibanyan crashed into each other, forming Buchinyan. However, unlike the first time they became Buchinyan, they became stuck together, so Shogunyan used a special attack to divide them. However, Whisper wound up crashing into Shogunyan, forming Whisshogun and getting stuck together instead.
Yo-kai Judge Bell
There's a Yo-kai that hosts the boxing competition between Rocky Badboya and Buttered Blue who resembles a judge's bell in EP095.
DisliKing's Retainers
They appear in Yo-kai DisliKing.
Naduzuke Yo-kai
Naduzuke Yo-kai (Japanese: 名付け妖怪 Naduzuke Yo-kai) appears in M02 when he tries to come up for a name for Fuu and Fuu 2.
He has greenish skin with the top half of their faces blue. They have grey wisp-like hair, pointy ears, fading blue gray stripes across their faces, a single tooth, and wear white collared long sleeve shirts with a black tie and a vest. Namer Yo-kai are usually seen holding a fountain pen and pale yellow notepads.
Camera Yo-kai
Camera Yo-kai (Japanese: カメラ妖怪 Kamera Youkai) appears in M02 when he takes a picture of Fuu 2's Yo-kai Medal.
Ninja Yo-kai
At the beginning of the third story in M02, a ninja Yo-kai delivers a letter from Koma Kaachan to Komasan and Komajiro's house with a sword.
Cat Yo-kai of the Overpass
After Jibanyan becomes a Yo-kai, he meets a trio of other cat Yo-kai called Pounce de Leon (Japanese: ネコ吉 Nekokichi), Sir Pouncalot (Japanese: ネコ次郎 Nekojirō) and Alfred (Japanese: さだお Sadao).
Whisper's Evil Spirit
Verygoodsir gives out a gift enclosed in a small package to Whisper in A Yo-kai New Year's Gift, which contains a black reflective crystal ball inside for "underperforming Yo-kai". To Nate, Jibanyan and Whisper's shock, a spirit emerges from it that looks like Whisper, except with a black body and evil eyes.
Whisper attempts to get rid of it, but starts to have nightmares in his sleep about his evil spirit scarily reminding him to do better as a Yo-kai butler otherwise he would be turned into a toy.
He briefly reappears in Yo-kai's Favorite Hit Music Station!, except that he turns evil in a last-ditch attempt to perform a song onstage before he was stopped by Rubeus J, Hardy Hound and Captain Thunder.
Repossessed Yo-kai
In EP116, Nate, Jibanyan and Whisper were curious about what lurked inside of Repossessor and explained to them about several Yo-kai he rejected because they never made it past the initial concept.
The Yo-kai they find are Chikyu Gomasan (Japanese: 地球ゴマさん Chikyū Gomasan), who is a blue komaniu enclosed in a half-spherical globe, Transfarmer (Japanese: トランスファーマー Toransufaamaa) who is an orange tank that can transform into a robot capable of farming, Unchiburiburi (Japanese: うんちぶり鰤 Unchiburiburi) who is an anthropomorphic red fish trying to defecate on a potty and Unmorashi (Japanese: うんもらし Unmorashi) who resembles Manjimutt except with light-blue underpants, yellow poop on his head and a censor bar over his eyes.
After Respossessor realizes that they need to get rid of Nate, Jibanyan and Whisper, they all get sucked in and meet more of the Yo-kai that were rejected whom resemble existing variants of Boss Yo-kai. They are Jason Dog (Japanese: ジェイソンドッグ Jeison Doggu) who looks like Rubeus J, Freddy J (Japanese: フレディ・J Furedi Jei) who looks like Rubeus J and Gashadokurobee-sama (Japanese: がしゃどくろべぇ様 Gashadokurobee-sama) that resembles Gutsy Bones.
Grand Prix Announcer
The Grand Prix Announcer appears in Yo-kai Grand Prix as the primary announcer for the Grand Prix along with Komasan and Komajiro, who announces the courses the contestants go through. He resembles the Song Medal announcer from the previous episodes.
Nyuso Yo-kai
In Yo-kai Injurnalist, there is a girl Yo-kai (which is called Nyuso (Japanese: ニューソ妖怪 Nyuso youkai) in the Japanese credits), who was seen in the front page of New Yo-kai Times, where it's stated that police were on the look out for a Yo-kai responsible for fake marriages.
Iekabe
Noway and House Partay accidentally fuse each other as Iekabe (Japanese: 家カベ Iekabe) in EP129 when Hailey and Usapyon builds a giant wall for Noway to break it in order to fulfil his dream when he was still alive.
BBQvil's Mother
During the barbecue party at Katie Forester's house, a Yo-kai who is the mother of BBQvil appears in EP132. She reminds him to stop eating meaty grilled foods and have grilled vegetables instead, which as a result inspirits the humans around them to do so, much to Nate's disappointment.
Shipshape Sailor's Crew
In EP138, it’s shown that Shipshape Sailor has a crew located on the inside of his head that controls the cannonball fires.
Brenda and Knomad Koma's Children
In EP171, Brenda appears at a television studio where she pitches a spin-off series of her and Koma Knomads that have children. Though their real names are most likely unknown, they're referred to in the credits as Eldest Son (Japanese: 長男 Chōnan), Second Son (Japanese: 次男 Jinan) and Third Son (Japanese: 三男 San'nan).
Android Pesu
In EP184, Sighborg Y breaks into his grandfather's mansion to look for his relatives and comes across a doberman guard dog. He uses the belly beam in the form of combat, transforming the dog into Android Pesu (Japanese: アンドロイドペス Andoroido pesu).
Unnamed Popular Cat Yo-kai
Appears in EP197, uncredited.
Mongewhale
A giant floating whale with Komasan's head called the Mongewhale (Japanese: モンゲーホエール Mongēhoēru) appears in EP200, where he grants wishes of others.
Un-evolved Eterna
An un-evolved Yo-kai, who is revealed to be Eterna without her stick makes an appearance in EP206.
Gyan Amakano
Gyan Amakano (Japanese: ギャン・アマカーノ Gyan Amakaano) appears in EP201, as Are Bacchino's right-hand man. He orders Yodelsen to barrage them with his beams, while taking photos of them with his phone. Amakano then commands Yodelsen to turn Cindy Jaws into a sugar cane, much to the shock of Indiana Jaws and his team.
Amakano has Yodelsen suction the heroes into his inner world in the book, in which he orders him to turn the page to a medieval story, which turns Amakano into a Dragon-like creature. With this new form, Amakano attacks the heroes by breathing fire, and when Indy and Jibanyan attempt to attack him, Yodelsen turns the page into a pirate story, which causes Amakano to reappear in a kraken-like form and attack them. However, the adventurers realize the presence of the Treasure Emblems in each of the stories in the book and collect them, and as Jibanyan rushes to get the last one in an Arabic-story, Amakano, in a Middle-east version of himself, tries to impede his progress. However, Jaws shoots him with his Yo-kai Magnum, which allows Jibanyan to succeed, and with all the Emblems collected, Yodelsen is weakened and Amakano and Are Bacchino meet their downfall when the heroes use the combined power of the Treasure Yo-kai against them, leaving them imprisoned in the book.
Yo-kai Watch Shadowside
Yo-kai Watch!
Shirokumama
Shirokumama (Japanese: しろく魔魔) is the mother of Shirokuma that appears in YW012.
Other Endangered Yo-kai
In YW032, It was revealed that Hailey Anne has been hunting for Endangered Yo-kai to keep them safe for extinction. While she had caught Nyabarukunina, she caught three other extinct Yo-kai.