Aldrich, Devourer of Gods (Dark Souls III) is strongly implied to have assimilated the frail remains of Gwendoline, Nito and Priscilla, creating the form that he has now.
Android 21 (Dragon Ball FighterZ) evolved and obtained an entirely new level of power after absorbing Cell.
Mira (Dragon Ball Xenoverse) ascends to his Final Form by absorbing his creator, Towa, and Tokitoki's egg.
Cell (Dragon Ball Z) is able to increase in power by absorbing others...
...and obtained his perfect form by absorbing Androids 17 and 18.
Franmalth (Fairy Tail) can Evolve by absorbing the soul other beings.
Meruem (Hunter X Hunter) consumes the flesh of a Nen user, assimilating their aura and powers.
Dehaka (Starcraft) is a notable Primal Zerg who can evolve by assimilation.
By repeatedly consuming other Ghouls, Ken Kaneki (Tokyo Ghoul) assimilated enough Rc cells to mutate into a powerful Kakuja.
Kapellmeister (Marchen Awakens Romance) transformed into a demonic being after absorbing his own Ghost Chess henchmen.
The Goliath (Evolve) is one of the monsters that are able to gain life, armor, abilities and grow in power by eating other life forms.
By continually absorbing/consuming other monsters, Alucard (Rosario + Vampire) evolved into a creature powerful enough to destroy the world.
Eaters (Digimon) are named such not just because of their endless hunger for data, but also because they can evolve without limit through assimilation.
An Eater that has assimilated multiple humans therefore becomes much more humanoid in appearance.
Aaroniero Arruruerie (Bleach) is a unique Hollow (now an Arrancar) who gains all the powers, memories, and skills of any Hollow he cannibalizes, thus allowing him to evolve endlessly.
Ikomikidomoe (Bleach) can evolve endlessly by breaking down and integrate the full spiritual power of anything it devours, growing exponentially stronger without limit.
Pernida Parnkgjas (Bleach) can, amongst other methods, keep evolving through assimilating other beings into itself.
Predator X's (Marvel Comics) genetics were left intentionally incomplete, allowing its DNA strand to evolve with every meal. This allows for increased size and to form defensive attributes against powers similar to its victim's.
Gourmet Cells (Toriko) evolve when their user consume delicious foods that suit them and the 'tastes' of the cells, enhancing the strength of the user and their abilities.
Bass.EXE (Mega Man Battle Network) evolved and obtained godlike power by absorbing the deleted data of viruses and other Navis.