Nega Tox turns into a Nega Seraphim at the end of the Nega Tox/Arnold Layne Arc in Mall Fight 3.
All angels and demons have this ability in Shadowhunter Peril. Greater Demons usually have command over an element or emotion, so whatever they transform into, their One-Winged Angel form will probably somehow represent that. Angels are the same. Some can't control their transformation, some can change at will, and others can only unlock their other form by unknowingly doing something specifically right. Of the angels and demons that appear in Shadowhunter Peril, only a few forms are known and have actually been seen:
Umbra Darkholme's true demonic form is eleven foot tall, metal-skinned abomination with massive shoulder spikes, and a skull fused around his face like a mask. His hands are claws, and whenever he transforms it is involuntary, as this tends to unleashes a Super-Powered Evil Side because he attempts to kill anything he sees, whether they be friend or foe. Since his transformation from demon to angel, it can be assumed his form has changed, but no one knows what it is.
Oblivion's form is more disturbing. His skin becomes black and hard as stone. His mouth becomes full of shark teeth, and he has four glowing red eyes. His cloak morphs into giant bat wings, and his entire body lights itself aflame. Even more terrifying is that he can change at will.
Lilith has yet to display a demonic form, but she's hinted at it a few times. When she gets seriously ticked off, her eyes turn into writhing snakes, and her blood becomes either acidic or it springs up multiple lesser demons to fight with her.
Nicholas also has yet to display his true angelic form, and judging from the level of power he had when he was in his relatively human form, everyone else is too scared to see what he
TVTome Adventures seems to love this trope, as almost all members of the D-Bug Organization do this in Season 2.
Rockoon also does this after hacking himself into the Shadowguard beast in the reboot.
from both the original version and the reboot, Alpha and Zetto's demon forms could count too.
Also in the reboot's season 2 finale, Rubirules does this when fusing with the antivirus, after Alpha does it with Kajet (the Forbidden Power).
At the end of Journey of the Cartoon Man, Oswald Sherzikien, facing defeat, consumes some of his own transformative ink and becomes a red-eyed cartoon giant.
Most lieutenants of the Vampire Kings in Tales from My D&D Campaign have these, such as a ginormous zombie worm which can disguise itself as a pudgy man. Another example is Morduval, an extremely decrepit old man who transforms into a giant tentacle-monster every time he attacks or is attacked.
Dreamscape: Vampire Lord can turn into a gigantic purple monster, which is Vampire Genesis.
Keedran's true form is an angel...mermaid...dragon thing.