Perhaps one of the more underrated powers in any anime is shapeshifting, the ability to change one's mass or otherwise assume the form of a different thing or species. While most would attribute this ability to nature-based heroes, such as Beast Boy of the Teen Titans, the fact is that plenty of anime characters can shapeshift.
Anime also has plenty of pure animal characters. These characters are usually intelligent enough to accompany the protagonist(s) on their journey and even provide aid in times of crisis.
Disclaimer: This list will contain spoilers as to many of these characters' fates, as well as true roles throughout their respective series. This list is also bound by the author's opinion.
5 Shapeshifters across various anime
1) Yoruichi Shihouin (Bleach)
One of Ichigo's primary mentors in the supernatural shonen anime Bleach is Yoruichi Shihouin. Acclaimed as one of the fastest Shinigami in Soul Society, Yoruichi can shapeshift in and out of a cat form. She usually uses this ability to remain inconspicuous since nobody usually suspects cats of espionage.
In comparison, even her top students, Byakuya and Sui-Feng, are still no match for her even after 100 years. She's never fought with weapons and only used armor against Aizen. In her supermode, she grows a cat's tail, pointed ears, and even claws made of lightning energy. The only drawback is that it reduces her to a feral state. She was poisoned by Askin and is confirmed to have survived via light novel sequels to Bleach's Thousand Year Blood War.
2) Orochimaru (Naruto)
The slippery, snake-like Orochimaru from the Naruto anime easily makes the list. Not only can he turn into a giant snake, but multiple snakes, and even other people, almost easily. It helps that this infamous anime villain has dedicated his life to studying various jutsu, including forbidden ones that help raise the dead.
Orochimaru has evaded death so many times it would almost be unbelievable without the pattern of Naruto's character. Orochimaru, in particular, has shed his skin like a snake, summons snakes, and has even appeared as a bizarre and twisted amalgamation of snakes. He was killed by Sasuke mid-way through Shippuden but was brought back since under heavy guard as of Boruto.
3) Qrow Brawnwen (RWBY)
One of the most drunken masters since Jackie Chan and Bo Rai Cho, Qrow Brawnwen of RWBY fame is never seen without a drink. Considered an excellent spy and Huntsman despite this vice, Qrow gained the ability to shapeshift into his namesake via a gift of magic from Professor Ozpin before the series began.
Qrow has shown the ability to use this gift to scout, spy, and get a literal birds-eye view of any battlefield or situation. This comes in handy multiple times throughout RWBY, whether trailing Team RNJR in volume 4 to keep them safe or reckoning a giant mech for weakspots in volume 6. Unlike his sister Raven Brawnwen, he doesn't consider the ability a curse. He's also ditched drinking since volume 7.
4) Oolong and Puar (Dragon Ball)
Considered a comic relief than anything else, Oolong and Puar of Dragon Ball still count as shapeshifters. While Oolong was only ever after money, Puar usually helped Yamcha when he was a bandit in his bid to steal coins or food. The two may not be as relevant since Dragon Ball entered Z, but it still counts.
These characters have used their abilities to help on occasion. Oolong has a limit of five minutes at a time for his transformations, and Puar can maintain his form indefinitely. Puar and he had to morph into bats to stop Pilaf from making a wish. Puar defeated Fangs the Vampire in Dragon Ball by turning into a giant hand.
5) Rimuru Tempest (That Time I was Reincarnated as a slime)
One of the most overpowered protagonists in an isekai anime, Rimuru Tempest, began his journey as a simple slime enemy when he reincarnated. However, as his knowledge and power grew, he updated his powers and tricks. One of them is shapeshifting. Not only does he have his slime form, but a human form as well.
In addition, Tempest can transform any part of his body into whatever he wishes with hardly any effort. This means he could make his hand an axe or sprout wings, among other valuable skills. He's also been able to disguise his appearance, as this is an extra skill on the Mimicry tree. Tempest is well known for becoming a literal god in his anime.
5 Pure animal characters across anime
1) Luna and Artemis (Sailor Moon)
The two cat guardians of the Moon Kingdom were stuck in suspended animation following the destruction of the Moon Kingdom. Once they awoke, Artemis and Luna began their search for the Sailor Guardians. The two are sentient, can speak, and can awaken the Senshi and provide useful items and advice.
Artemis got to Minako Aino first, awakening her as Sailor Venus and helping her for over a year. These adventures are chronicled in the Codename: Sailor V manga. As for Luna, she helped awaken Sailor Mars, Mercury, Moon, and Jupiter. Both serve as "mission control" for the Inner Senshi, briefing them on monsters, keeping an eye on the city, and even helping motivate the Senshi. Usagi especially needed it on her first day.
2) Kerberos/Cerberus/Kero (Cardcaptor Sakura)
Kero roped Sakura Kinomoto into becoming a Cardcaptor after she accidentally set loose demonic cards called Clow Cards in the magical girl anime Cardcaptor Sakura. It didn't help that Kero was asleep in the book containing said cards for over 30 years. Very much unlike being stuck in suspended animation.
He might have two forms, but they're both animals: a winged lion cub for his first form and a large maneless helmeted lion or cougar with enormous white wings and a breastplate with a red jewel. He gives Sakura her Sealing Wand and helps with fire breathing and energy barriers. He also has a good sensing ability for detecting Clow Cards and a sweet tooth that even puts Sailor Moon's to shame.
3) Kyubey (Puella Magi Madoka Magica)
This deceptive, vile, and evil entity is part of an alien race called the Incubators. Kyubey appears cute and cuddly, but it's a trick to deceive audiences and their intended victims. Kyubey is helpful about awarding powers to magical girls and gives some advice about fighting witches, but that's the only good thing he does.
In reality, his race is trying to prevent the literal heat death of the universe. They do so by harvesting the energy of teenage girls by transforming them into magical girls and inevitably turning them into witches without any emotion or realizing why it's wrong. Kyubey understands the consequences as his system finally shatters when Madoka becomes a goddess.
4) Kirara (Inuyasha)
Onto more adorable traveling companions, Kirara from the shojo anime Inuyasha started as Sango's loyal nekomata and mode of transport. While Kirara can morph into a larger, saber-toothed cat, she is still a cat demon and, thus, an animal at the end of the day. She's also not to be trifled with, as she's able to fly when transformed, use fire, is immune to fire, and has enhanced speed, durability, strength, and endurance when transformed.
As for the role and aid she brings, she can easily carry three fully grown adults on her back when transformed. She's also cat-like, loves cat food, treats, and is very intelligent. She helps the others in their own quests: Miroku with impressing village women, Inuyasha with attack practice, and even Kagome with transport back to the modern era. She joined Kohaku as his traveling companion three years after Naraku's defeat.
5) Pikachu (Pokémon)
Animal companions and Pokémon are practically synonymous, and the anime version of Pikachu is an icon in this regard. Starting as a rather rebellious Pikachu in Kanto, Ash and his Pikachu did not get along initially. From random thunderbolts to refusing to get into a Pokéball, Pikachu seemed determined to make Ash's life difficult.
However, this seemingly diminutive electric mouse is not one to be messed with. Pikachu has shown enough power and resilience to fight off opponents twice his size and three times his weight. It helped that he and Ash started to get along by Brock's gym, and by the time the original anime got to the Johto region, the two were best friends.