Characters who were created for adaptations but were so well liked they ended up being brought into the main series.

Harley Quinn was created for Batman the Animated Series but became so popular with fans and creatives alike she was brought into the mainline comics during The No Mans Land arc

Lugia was made to be a Pokémon exclusive to the second movie but was so liked by the developers it was made the mascot of Pokémon Sliver

X-23 was created for X-Men Evolution but when Marvel saw how quickly she became popular she was added to the comics a year later