Hell Teacher Nube (Yako/Fox Transformation)

In episode 2, Tamamo reveals his true self as a yako (fox spirit) before he completes his transformation into a human:

Digimon Ghost Game (Demon Transformation/Reversion & Size Growth)

Episode 39 ("Contagion Island") involves a little monster who likes infecting people. This makes him grow bigger and turns his victims into demons (they look like "Gyuki" yokai). The main transformation in this episode involved a female character, but thankfully there was a male reversion scene. In this clip it's preceded by the infection scene and the monster's size growth.

Yokai Watch (Monster & Muscle Growth Transformations)

Yokai Watch is a gigantic video game/anime franchise and keeping track of all the different anime within it can be difficult. This first clip is from episode 34 of Youkai Watch Jam: Youkai Gakuen Y - N to no Souguu and shows a loser turning into a hideous monster called "O-Eater":

The second clip is from episode 5 of Yokai Watch Shadowside and is supplied by Male Muscle Growth Archives, whose description I quote: "Jinta, a medical doll yokai, is corrupted into a monstrous hulking titan": 

Undoubtedly more transformations from this franchise will pop up... 

BEM: Become Human (Demon & Monster Transformations)

 BEM: Become Human is the spin-off film of the anime series Humanoid Monster Bem. The clip below is a supercut of all the transformation scenes in the film. Since it's lengthy, I've provided a list of the scenes.

0:00 A thug transforms into a yokai (Japanese demon).

1:21 The protagonist Bem starts transforming into a yokai.

1:48 Bem's fake children transform into yokai, while the villain taunts him on the phone. After slaying the monsters, Bem goes outside and finds the townsfolk turning into yokai as well.

3:28 Bem enters the villain's headquarters and fights a yokai security guard.

3:40 Bem confronts one of the secondary villains, who turns into a metallic monster.

4:45 Bem meets with his young ally Belo, who can also turn into a yokai (a good one).

4:54 Bem finally confronts the main villain. He fully transforms into a yokai, but the villain responds by turning into a giant white monster. 

6:03 Bem's yokai form grows more monstrous.

Note: We plan posting the TF scenes from the series as well, but they're mostly computer morphs and thus inferior to the 2D animated transformations in this clip.