Superman: The Animated Series (Monster Transformation)

In episode 10 of this classic cartoon, a villain known as "The Preserver" reveals his true form. Thanks to Andréssaurio Rex for the link. 

Fire and Stone: Prometheus & Aliens vs. Predator (Alien and Monster Transformations)

Thanks once more to Andréssaurio Rex for suggesting this crossover comic series, set in the Alien vs. Predator franchise. (Spoiler warning.)

In Prometheus: Fire and Stone #02, the scientist Francis, who is dying of cancer, convinces Elden, an android, to be injected with a mysterious alien "accelerant," in order to test its possible regenerative effects:

Issue three finds Elden, now a mix of machine and alien, resenting his new form:

 In issue four Elden sets off into space to get revenge on Francis:

 

In Alien vs. Predator: Fire and Stone #01 Elden works with the xenomorphs to corner Elden:

 

Issue 2 finds Elden fighting and biting a Predator, who undergoes a transformation of his own:

Issue 3 opens Elden getting attacked and bitten by Xenomorphs, causing him to mutate again:

Francis, hoping to cure his cancer, steals some of Elden's blood and injects himself with it. Of course a hideous monster transformation ensues:

Issue 4 opens with the still-transforming Francis getting attacked by the mutated Predator and rescued by Elden. The two reconcile, only for Francis to start dying--the accelerant has sped up his demise:

In the final chapter of the crossover, Prometheus: Fire and Stone—Omega, Elden crash lands on an alien planet and merges into a living mountain of alien flesh:

 

HumanKind (Monster/Alien Transformation)

Thanks to Dinonator for finding this miniseries from 2004. The first issue shows a captured astronaut turning into an alien-monster hybrid:

Issue 4 delivers more transformations and reversions:

The Orville (Arachnid/Alien Transformation)

Thanks to MrTF for finding this scene from The Orville (season 3, episode 2). A member of the spaceship crew gets ambushed by an "arachnid," a spider-like alien, and transforms into one:

Assassination Classroom, Wai Wai Quick Noodles, and The Extant (Monster and Alien , and Age Progression)

My thanks to Ian Ricky for finding and uploading the following clips. 

From the Japanese film Assassination Classroom: Graduation (2016), Yanagisawa (Hiroki Narimiya) injects himself with artificial tentacles and turns into a big monster:

Next, a human reveals his true alien self in this commercial for Wai Wai Quick Noodles:

And for those who'd like to see even more skin-shedding, here's an age progression scene from The Extant (S02E03):

Animorphs: The Message (Fish, Dolphin, Shark, Seagull, and Alien Transformations)

In this update we bring you scans from two Animorphs graphic novels. This post showcases volume four, The Message. Go a few posts down to find volume five. 

The Message shows our heroes (the two males are Marco and Jake) undergoing various aquatic transformations. They start by turning into fish (salmon?):

Later they turn into dolphins, but Marco gets injured and has to revert to human form:

 

Then comes a scene where everyone reverts from seagulls to humans:

And in case you're getting sick of reversions, here's a scene of Marco turning into a dolphin:

Followed by everyone going from dolphins to humans:

Our heroes are joined by an alien named Ax. Together they escape from the bad guys by morphing again: the humans into dolphins and Ax into a shark.

Lastly, Ax decides to morph into a human, but needs help getting dressed: