This Isn't Even My Final Form

This Isn't Even My Final Form is a trope common in Shonen Action (Battle) series and a broader meme that's popular in the anime and related media fandom. This expression is used to denote the trend of antagonists in combat focused anime changing their appearance and growing more powerful when pressured in a fight, thus creating more narrative tension. The phrase is widely attributed to the Dragon Ball villain Freeza, who undergoes various transformations during his first battle with the series' heroes, however the character does not utter the expression in any iteration of the series.

History
As a trope, villains transforming into a more powerful form is one of the oldest and most common plot devices in shonen anime and manga. Especially in Mecha series where transformations are commonplace, this action is usually used by one participant in a fight to gain the upper-hand against the other combatant. This kind of transformation is distinct from the Magical Girl transformation, as it involves a superhuman character becoming even more powerful, as opposed to a grounded character gaining supernatural abilities.

Notable villains that transform in this fashion include Toguro from Yu Yu Hakusho, DIO from JoJo's Bizarre Adventure, and Father from Fullmetal Alchemist: Brotherhood.

As A Meme
"This Isn't Even My Final Form" grew in popularity as a meme and phrase in the otaku community beginning in the early 2010s. Most often, this meme takes the form of image macros of people in elaborate dress or cosplay, strangely modified models or sculptures, or enraged individuals. This meme is also used fairly often in the Pokémon fandom and other communities centered on RPGs, as character growth and transformations are usually integral to those properties.