In 3-2-1 Penguins!, out of the six main characters, Michelle is the only female character.
Adventures of the Galaxy Rangers had a team of four elite agents of the Bureau of Extra-Terrestrial Affairs. Out of the eight recurring characters from BETA, only Niko is female; the rest of the organization is male, a male-identifying android, or dolphins.
Attack of the Killer Tomatoes! Tara is the only female tomato in the show (of course, she looks as a normal beautiful woman if not in contact with salt). All other tomatoes (both humanoid and traditional-looking) are male. She's also the only female in the main cast, but among the recurring characters there are other women.
The first season had only one female character from episode to episode; Katara, while the other heroes, villains, and vast majority of recurring minor characters were male. The second season added more female characters, including another girl joining the core group, making an even ratio of two males to two females, not counting the (male) team pets, Momo and Appa. Early production notes for the series indicate that everyone besides Katara was male in the original draft. Remember that large male earthbender in the opening who never appeared in the show? That was supposed to be Toph.
During the introduction episode of Toph Beifong, Earth Rumble VI was a sendup of Professional Wrestling, so all of the contestants and audience were males; when they introduce "The Blind Bandit", no-one seemed to know she's actually a girl.
Avengers, Assemble! has the Black Widow main character as the only female hero on a team with eight members, the others being Ant-Man, Iron Man, Thor, Hulk, Hawkeye, Falcon and Captain America. Season 4 adds 3 new female characters (Captain Marvel, Ms. Marvel, and The Wasp) on a team of thirteen members.
In The Avengers: Earth's Mightiest Heroes! Wasp was the sole female Avenger in season 1. Averted by season 2 adding the titular character from Ms. Marvel to the cast. Other females appear in less prominent roles throughout the series, keeping the trope limited to the 8-person team in season 1.
Boo Boom! The Long Way Home: Aurelia the cat is the only female member of the 5 animals that accompany Boo Boom, and for a large part of the series also the only female in the entire main group. That last one gets averted during the episodes in which Sixth Ranger Viola is present.
Capertown Cops: Coco Loco is the only female of the Capertown cops, and is just as much a bungler as the others.
Chip 'n Dale Rescue Rangers: Chip and Dale are brothers, who pick up Montery Jack and his pal Zipper. Rounding out the crew is the only female; Gadget (who was actually the daughter of the guy they wanted to recruit).
The Defenders of the Earth consist of seven males and one female, the latter being the Phantom's daughter, Jedda. Happily, there are a few more female characters in the supporting cast (including Jedda's panther).
On Dragon Booster, the main cast is made up of three males (Artha, Parm, and Lance), and one female (Kitt). Though initially a rival to Artha, Kitt became a cheerleader for him and consistently beaten in any kind of race (despite the fact that she had more experience at racing than Artha, who didn't want to race at all at the start of the series). There were other female supporting characters, including a few crew leaders, all of whom were present to support the males. The "Kitt can never win" issue might have some connection to the advertising assumption that you can't show a girl winning a board game, or it'll be less appealing to boys.
Gargoyles begins with Elisa Maza (a human) as the only main female character, balancing the five (male) gargoyles and their gargoyle beast. As the show developed, more female characters show up, including Angela (Goliath's daughter) joining the Manhattan Clan.
April is the only girl in The Funky Phantom, stuck with her two friends Skip and Augie (always feuding), their pets Elmo and Boo (always feuding), and the Revolutionary war ghost Jonathan Muddlemore (Mudsy for short).
The animated series of The Little Rascals consisted of four boys and Darla Hood, although this is probably a legacy from the original shorts, where the boys were firmly in their "Girls Have Cooties" stage.
In the original series, Jezebel Jade was the only female character to appear in at least two episodes. Several episodes had all-male casts.
The 1980s version introduced Jessie Bradshaw to the all-male cast of the original series. She was meant to be a recurring character, but only appeared in the last episode before the series' cancellation. She would return in the two follow-up made-for-TV animated movies, which retconned/revealed her to be Race Bannon's daughter. In the 1990s update, she was made a main character.
Penelope Pitstop was the only woman in The Perils of Penelope Pitstop; her own series! Her friends in the Ant Hill Mob, a group of seven little men, would rescue her from the Hooded Claw's traps (and she'd then rescue them).
For the first season of Shirt Tales, Pammy Panda was the only female in the group. When the show began its second season, a female kangaroo was added to the cast.
Penelope Pitstop was the only woman in Wacky Races (a series that featured 11 different racing teams, averaging two racers to a car).
Hero: 108's Mystique Sonia is the only female of First Squad, the other 4 members (including a rabbit) being male.
On Jimmy Two-Shoes, Heloise is the only female in the final group shot at 5:1. Recurring characters Saffi and Jez ease this a bit, but they tend not to have much of a role.
The male cast of KaBlam!! (that was included in over three episodes) consisted of Henry, Mr. Foot, and Mr. Stockdale (starting Season 4). "Over three appearances" girls? June.
Kaeloo's first season has only the title female character, with three male characters. In the second season, however, new characters Pretty, Eugly, Olaf and Olga note Olga, despite being a Companion Cube, is counted as a character are introduced, creating a Gender-Equal Ensemble with four males and four females.
Kim Possible has Team Go, the superhero family that Shego is from. Her brothers are "Hego", "Mego", and "Wego".
Legion of Super-Heroes started the show with female Legionnaires Saturn Girl, Phantom Girl, and Triplicate Girl. And then, in Season 2, the powers that be decided that male viewership would be put off by so many girls, so the girls were incapacitated and/or inexplicably sidelined for many episodes. The Legion has Loads and Loads of Characters with a fairly even gender —and species— balance, so the comics were averting this trope even all the way back to the Silver Age! The addition of Shrinking Violet in the same season was a very small counterbalance.
Fuli from The Lion Guard is the only female of the titular group. Kion, Beshte, Ono, and Bunga round out the rest. There are a few additional females who are not part of the Guard, so Fuli isn't completely alone.
Motorcity: Protagonist organization the "Burners" consist of Mike, Chuck, Dutch, Texas, Jason and Julie (the only female), plus R.O.T.H. ("male" robot). She's at least accompanied by Tennie, Foxy and Kaia in context of the wider cast.
Ōban Star-Racers: The main character is Eva/Molly, a female mechanic for the human team. Don Wei, the owner of her racing team, believes that women shouldn't be racers. However, Maya Wei shows up frequently to affect the story by providing motivations for both of them.
PAW Patrol started with only one female team member, Skye. Everest was added as an occasional Sixth Ranger in the second season.
In Rambo: The Force of Freedom, Kat is the only major female character in the series, which consists of at least five other characters.
For the entire first season of Regular Show, there was only one female in the cast: Margaret the cardinal, who wasn't even part of the eight main characters. She only appeared in three episodes, one of which was only via dream sequence. However, Season 2 has introduced another female character, though both are supporting cast members.
Janine in The Real Ghostbusters is the female secretary to the four-man team of ghost busters (although she occasionally help out when the chips are really down).
The title team of Road Rovers consists of five male members (Hunter, Blitz, Exile, Shagg, and Muzzle), and one female (Colleen).
In Robotix, Narra is the only female Protecton as well as the only female character in general. While Compucore speaks in a feminine voice, they are technically genderless, considering they are a computer.
Rock Dog has eight main characters, only one of which (Dharma, the fox bassist) is female. There are, however, more minor female characters.
Rocko's Modern Life began with a cast of five recurring characters (including Rocko the main character), the only female of which was the wife of the Cranky Neighbor; Bev Bighead. Averted when the creators added Affirmative Action Girl hook-for-a-hand-pirate-style Dr. Hutchinson as a love interest for Filburt. They wanted a female with a hook, you see.
Lampshaded on a "fan club" segment on Rocky and Bullwinkle, where the club holds a picnic with a "prettiest girl" contest. Despite having a cast of five (Rocky, Bullwinkle, Fearless Leader, Boris, and Natasha), Natasha wins by default because she's the only female member.
Of course The Smurfs originally had only Smurfette, in a cast of dozens of male smurfs. The trope namer would avert this with the additions of Sassette and Nanny Smurf.
Smurfette wasn't originally part of the series. She was a black-haired creation of Gargamel and meant to be used to cause discord among the Smurfs. She became a blonde when she became a 'real Smurf'.
South Park: Played with in the "You Got F'ed in the A" episode of South Park, when Stan is putting together his dance team.
Street Sharks had a cast of four teenage boys transformed into sharks, and their five friends (some of whom were also transformed into fish-people). But the only female in the group was Lena, a friend of theirs from college.
In most episodes, Wonder Woman was the only female hero, particularly egregious during the Challenge of the Super Friends season, when the ranks of the Super Friends swelled to 11 and she was still the only female. Even the Legion of Doom had two female members — and they would only use as many villains as there were heroes.
Superfriends did have some aversions by adding Wendy, or Jayna of Wonder Twins, Hawkgirl, or Rima the Jungle Girl, but they only appeared in a handful of episodes, and never more than Two Girls to a Team.
Superjail!: Alice is the main female protagonist, in a group of four, but also the only woman in Superjail other than the lunch ladies, who are much more minor characters. Her Spear Counterpart Bruce also appears to be the only man in Ultraprison. Of course, as both are stated to be Transgender, their status tends to be debated among fans, and the show plays with gender roles in relation to Alice.
In Teenage Mutant Ninja Turtles Adventures (published by Archie Comics and loosely inspired by the old cartoon), this principle was eventually averted by April being joined by Ninjara; both girls were trained in the ninja skills and active at the same time.
ThunderCats (1985) began with a cast of Lion-o, Panthro, Tigra, Snarf and Cheetara, with occasional appearences by Jaga. Only Cheetara is female, but the series would continue to add characters, including WilyKit and Pumyra.
Total Drama World Tour has Team Chris is Really, Really, Really, Really Hot, which consists of four (later five) guys, and Izzy (who switched teams with Sierra, so they could be with their love interests). The season also includes a team following The One Guy and a Gender-Equal Ensemble.
Transformers: For a long time, the male to female ratio was obscenely distorted, no matter where you looked. The excuse was that they're robots and have no gender, which doesn't hold up well when the male robots use gender pronouns for themselves.
The Allspark Almanacs have added a few more girls, but they also include the Omega Sentinel roster - out of twelve "Greek-letter-Supremes", only one confirmed female.
The Transformers Aligned Universe mentions that all female Cybertronians are descended from Solus Prime, the only female among the original Thirteen Primes.
While the original dozen of Beast Wars characters (Maximals and Predacons) were entirely male, female characters Blackarachnia and Airazor were introduced in the first season (to different sides). However with Airazor getting beamed into space by some freaky alien plants midway through the second season, Blackarachnia remains the only female in half of the second and the entire third season. Interestingly enough, Airazor does return as Tigerhawk, a fusion of both hers and Tigatron's bodies, but the character is presented as male.
The Transformers that appear in Transformers: Rescue Bots are mainly male, with the first female Autobot Quickshadow being introduced in the fourth season.
Visionaries has two teams of seven characters, each of which has only one female:
Xiaolin Showdown and its sequel Xiaolin Chronicles: Kimiko is the only female Dragon of the four (all three monks are male) on the Xiaolin side, this contrasts with Katnnappe, Wuya, Shadow, Clay's sister, and the evil mermaid for the Heylin (evil) side. This almost seems to imply that Kimiko is the only female member because she is the exception to the rule of females automatically calling for the side of evil.
X-Men: Evolution: Throughout the series, there is only one Brotherhood chick on the all male team at one time or another (the males were Avalanche, Toad, Blob, and Quicksilver). The first several episodes has Rogue before she joins the X-Men. Boom Boom joins them for most of the second season before she leaves. Then finally the Scarlet Witch joins and becomes the permanent Brotherhood babe.
The first several episodes have Robin, Kid Flash, Superboy, Aqualad, and Miss Martian (the only female) as the Young Justice team. Averted when Artemis joins up.
The robots built by T.O. Morrow have a bit of a theme to them; Red Torpedo, Red Tornado, and Red Volcano, are all androids, while Red Inferno is a gynoid (robot who resembles a female human; the Greek prefix "andr-" refers to the masculine gender).
The Injustice League (Black Adam, The Joker, Poison Ivy, The Ultra-Humanite, Count Werner Vertigo, Wotan, and Atomic Skull) were a seven-villain team that had Poison Ivy as their lone female member.
The Forever People (Vykin, Dreamer, Bear, Serifan, Moonrider) can summon Infinity-Man, but only Dreamer is female.
The Light, a group of seven villains, has only one female member; Queen Bee.