Locacaca Fruits (JoJo's Bizarre Adventure Part VIII: JoJolion)
Wall Eyes (JoJo's Bizarre Adventure Part VIII: JoJolion)
Talisman of Kamar-Taj (Marvel Comics)
Time Teleporter (Monster High: Freaky Fusion); through accident
Combine-Inator (Phineas and Ferb)
Ruber's potion (Quest for Camelot)
Transporters (Star Trek); through accident
Millennium Scale (Yu-Gi-Oh!)
Polymerization card (Yu-Gi-Oh!)
Orb Ring (Ultraman Orb)
Firestorm Matrix (Arrowverse/DC Comics)
Ben 10,000's Biomnitrix (Ben 10: Omniverse) can fuse any two Omnitrix aliens into one transformation.
In order to free himself from all his grief and emotions caused by him losing both his family and friends, Danny Fenton had Vlad split his human half from his ghost half, but freed from his emotions, his ghost half merged with Vlad's ghost half…
…Danny's ghost half filled with all his negative emotions was further corrupted through him fusing with Vlad's evil ghost half, resulting in the creation of the most powerful and sinister ghost to ever to exist on the planet…
…wreaking havoc across both the Human World and Ghost Zone for ten years, Dark Danny became the evilest and strongest ghost in existence.
The Fenton Ghost Catcher (Danny Phantom) can both separate ectoplasmic energy like ghosts from humans and merge them back together again.
The Crystal Gems (Steven Universe) are members of a race of magical aliens capable of fusing together with one another, creating different variants of fusion-beings.
Steven and Amethyst (Steven Universe) fuse to form Smoky Quartz.
Boxman's robots (OK K.O.! Let's Be Heroes) combined into Boxbot Prime.
Ruber (Quest for Camelot) uses a special potion to merge human beings with weapons...
...doing so with himself and Excalibur to ensure no one would ever take it from him.
Brainiac and Lex Luthor (Justice League Unlimited) fuse to become one being known as Luthoriac.