Maybe I should had done this a long time ago, but I guess Power Rangers Samurai has sort of propelled me to do this. Both Kakuranger and Shinkenger monsters were based on Youkai. The ones from Kakuranger were called Youkai but had 'modern influences.' The Ayakashi have a more complex look and has more to do with an animal.
Namiayashi and Kappa
Both based on the water sprite Kappa. Kappa looks more like a frog. Namiayashi has two sides and one of it looks like a tiger. Namiayashi is known as Doubletone in Samurai. Kappa appeared unnamed in Power Rangers.
Urawadachi and Ittanmomen
Both based on the Ittanmomen, Ittanmomen flies through in the air and attack people by wrapping itself around their face and throat trying to suffocate them. They don't look alike at all. Ittanmomen from Kakuranger appeared as a toy called Calicfire but not in the show. Urawadachi hasn't appeared yet in PRS, he looks like a hammerhead shark.
Okakurage and Kasabake
They look alike. Okakurage is based on Karakasa is the spirit of umbrellas, has one eye, a long tongue, a single foot, and umbrella head. Kasa-Obake is a spirit of parasol are typical portrayed with one eye, a long tongue and single foot that originated from objects reaching their 100th year of existence. Kasabake appeared unnamed in MMPR and Okakurage as Desparino in PRS. Both had water powers.
Oborojime and EnraenraAn Enenra is a yokai, or Japanese monster, that is composed of smoke. It resides in bonfires and, when it emerges, it takes the form of a human. It is said that an enenra can only be seen by the pure of heart. Enraenra appeared in toy form in MMPR but not on the show. Oborojime hasn't appeared in PRS yet. Enraenra looks like a robot with canisters and machines, while Oborojime looks like natural smoke.
Tsubotoguro and Rokurokubi
Rokurokubi resemble humans and can stretch their necks at night. They can also scare humans by changing their faces and are practical jokers. Rokurokubi was female, parts of her appeared in Power Rangers, Tsubotoguro appeared in PRS.
Sunasusuri and Gakitsuki
Gakitsuki is based on Gaki, which is also known as Preta, Preta are a type of supernatural being described in Buddhist, Hindu, Sikh, and Jain texts that undergoes more than human suffering, particularly an extreme degree of hunger and thirst. They are spirit of jealously or greed that cursed people with an insatiable hunger for a particular substance as punishment for their vices. Traditionally the substance could be something humiliating but in most recent legends, it can be basically anything.
Abekonbe and Kasha
Both based on Kasha, who is a cat-like monster that steals corpses during a funeral. Kasha is Miss Chief in MMPR and not yet used for PRS. Kasha looked like a fire fighter, hence the American name. Abekonbe had like a lion head on the chest and had a teapot-like body. He switched human's minds with ordinary items or food. Kasha had fire power.
Azemidoro and Dorotabou
A mud spirit that emerges from rice paddies' murky mud and while not completely dangerous, his appearance of aged, bald and apparently blind in one eye, it frightened people. Dorotabou of Kakuranger didn't appear in MMPR and the cauliflower-like fame-driven youkai who wanted to be on TV. Azemidoro may or may not appear in PRS and he looked for revenge.
Futagawara and Nurikabe
A spirit that manifests as a wall that misdirects or hinders walkers at night. Trying to go around the Nurikabe is gutile as it extends itself forever. Knocking on the lower part of the wall makes it vanish. Nurikabe is known as Brick Bully for MMPR, he looked like a brick wall. Futagawara looks much different, he appeared as Skarf in Samurai.
Sogizarai and Yamauba
Yama-oroshi is a radish-grater spirit, a pun on a word for 'mountain storm.' Sogizarai is known as Shark Jaw in PRS' "Clash of the Red Rangers". One of Yamauba's forms appear on MMPR like Dischordia and her other form appeared in background scenes. They look radically different.
Narisumashi and Nopperabou
Usually mistaken for a badger, Noppera-bo is a faceless ghost that frighten people, some believe human spirits disguised themselves as a scary form. Nopperabou could break apart and form again. Narisumashi could pretend to be other people. Nopperabou only appeared as a unnamed background character in Power Rangers. Narisumashi appeared in a Christmas episode.
Isagitsune and Tengu
Both based on the Tengu, although they take their name from a dog-like Chinese demon, the tengu were originally thought to take the forms of birds of prey, and they are traditionally depicted with both human and avian characteristics. Also the most popular of Japanese folklore. Isagitsune is called Vulpes in PRS and Tengu was General Longnoose in MMPR. Isagitsune had a fox-theme. Also the footsoldiers of MMPR season 3 were originally known as Tengu, based on the birds of prey, later renamed Tenga because of 20th Century Fox Films copyright.
Yumebakura and Bakuki
I'm not sure, I have to ask my friend, but Yumebakura is based on the Baku but looks like a Rhino. In PRS, he's known as Rhinosnorous. Bakuki is known as Vampirus in MMPR. Baku devours dreams and nightmares and looks like a tapir.
Ootsumuji and Kamaitachi
There are several conceptions of how it looked like or operated, but the most common is one of a trio of weasels with sharp claws, riding on a gust of wind and cutting peoples' skin on the legs. According to this interpretation, the first weasel knocked the unsuspecting victim down, the second cut the victim's flesh and the third applied medication to the wounds, so that at the time the victim realized what was happening they were left only with painful wounds that weren't bleeding. Ootsumuji is known as Scorpionic in PRS and Kamaitachi wansn't in MMPR. Ootsumuji had a weasel face and a scorpion or insect like armor.
There might be more, I know i missed some.