The "FernGully" forest depicted in this movie was based on Australia's rainforests. The cartoonists who worked on this movie spent time in the real rainforests to help inspire their drawings.
The original lyrics to the movie's song "Toxic Love", performed by Tim Curry, had obvious sexual themes/implications, and various verses from the original song were cut and or re-written to make the song more appropriate for a family movie. The original lyrics can still be found, but the song's full version was never used.
Tim Curry's original voice recordings were so eerie and sinister that it made children cry during test screenings. Curry had to be called back in for some less intimidating retakes.
Robin Williams provided 14 hours worth of improvised lines for Batty Koda. Director Bill Kroyer was so impressed with the voice work that he ended up tripling the screen time given to the character (which was originally supposed to be an eight-minute role).
In Spain, it was dubbed into Spanish entirely by comedian Ángel Garó, a single-handed feat that earned him entry in the Guinness Book of Records.