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 rewritten to make the song more appropriate for a family film. The original lyrics can still be found but the song's full version was never used.
Originally supposed to be released in November 1991 but the release was delayed to avoid the competition with Disney's Beauty and the Beast (1991). Instead, it was released in April 1992 and competed with Rock-A-Doodle (1991).
When Zak carves Crysta's name into a tree he only gets as far as 'Crys' before she stops him and explains the pain he is causing for the tree. Although misspelled, stressing the word 'cry' is certainly a reflection of the tree's pain.
Elton John's 1st animated feature film which he contributed to the film's soundtrack. He performed the end credits song "Some Other World". Elton John later collaborated with Tim Rice on the soundtrack of The Lion King (1994) and composed the soundtrack of Gnomeo and Juliet (2011) which he performs the song "Hello, Hello" with Lady Gaga.
Tim Curry (Hexxus) and Robin Williams (Batty Koda) have both worked with director Chris Columbus. Curry; Home Alone 2: Lost in New York (1992). Williams; Mrs Doubtfire (1993), Nine Months (1995) and Bicentennial Man (1999).
A common criticism the film faced, even to this day, is despite the film's message of saving the trees, it was an animated film. Where trees would obviously have been used to make the paper to animate it.
Christian Slater and Robin Williams have both worked with director Barry Levinson. Slater; Jimmy Hollywood (1994). Williams; Good Morning, Vietnam (1987), Toys (1992) and Man of the Year (2006). Coincidentally FernGully: The Last Rainforest and Toys were both released in 1992.
Robin Williams provides the voice of Batty Koda, a wacky mentally unstable bat that was experimented on by humans. Robin Williams almost got cast as The Riddler in Batman Forever (1995) which Jim Carrey was cast.
Consecutively this is another film in which Christian Slater has "starred" - as he voices an animated character - in a film featuring a type of forest. The previous year Slater starred in Robin Hood: Prince of Thieves (1991).
The trivia items below may give away important plot points.
The later 20th Century Fox film Avatar (2009) was accused online of ripping off FernGully: The Last Rainforest (1992) due to similarities: In both films, the moronic human character are physically transformed into interact with the intelligent humanoid species of a forest who live in a special tree which his employers seek to destroy. The hero gets lost and chased by a purple predator and is saved and falls in love with the tribe leader's daughter, she teaches him to feel the pain of nature and that it is connected, whilst they have a romantic swims and jump around in a sparkly florescent forest. Along the way he competes with the local alpha male and mourns the death of a older female mentor and it all ends in a aerial battle with massive human machines and nature wins.