In the original A.A. Milne books, heffalumps are depicted as figments in Pooh's nightmares. In the first book, as heffalumps are eating honey within his dreams, Pooh Bear becomes deeply distressed and wakes up to find that it never truly happened. In The Many Adventures of Winnie the Pooh, a very similar instance occurs after Tigger introduces both heffalumps and woozles to Pooh very shortly after they meet. Because of Tigger's words, Pooh Bear stays up throughout the night, guarding his honey. After hours of staying awake, he lulls to sleep in which, much like the first book, he dreams of heffalumps, and, this time, woozles as well. While he does not look deeply distressed by the nightmare itself, Pooh does show signs of uncomfortableness all throughout. When he wakes, he speaks to himself in the mirror and never mentions this to any of his friends.
Heffalumps are not mentioned again until the first season of The New Adventures of Winnie the Pooh. In the episode "There's No Camp Like Home", Piglet also shows deep distress while experiencing a heffalump and woozle induced nightmare of his own. After reluctantly going camping with Tigger and Pooh, he soon meets a family of heffalumps who, while obsessed with honey as past Winnie the Pooh literature and animation suggests, are not necessarily evil. This and the next episode they appeared in, "Trap as Trap Can", suggest that not all heffalumps are to be feared, though the extreme desire for honey still remains evident. When the episode "The Great Honey Pot Robbery" was released, we're introduced to another heffalump named Heff, who, much like the heffalumps depicted in Pooh and Piglet's nightmares, is a villain who chose to associate himself with a woozle named Stan.
After the appearances of the heffalump family and Heff himself, actual heffalumps are not seen again in any Winnie the Pooh animation, except as intimidating figments, as seen in Tigger's song sequence in Boo to You Too! Winnie the Pooh, a Halloween special that aired in 1996. While they are indeed mentioned by the main characters in other Pooh works as well, none are physically seen.
In 2005, Pooh's Heffalump Movie brings heffalumps back and are not only physically seen, but, as the title suggests, they are the main focus of the movie. In the film, Pooh, Piglet, Tigger, Eeyore, Rabbit and Roo are introduced to heffalumps. They fear them as strange creatures with "fiery eyes" and "tales with spikes." They are initially intimidated by one, an innocent kind-hearted one named Lumpy, but Roo becomes friends with him. After a misunderstanding, the others soon warm up to him as well, introduce him to Christopher Robin, and are soon seen interacting with other members of the heffalump community. Lumpy continued to appear in Winnie the Pooh media, including Pooh's Heffalump Halloween Movie and in a semi-regular role in the series My Friends Tigger & Pooh.
Before the release of Pooh's Heffalump Movie, heffalumps were mostly seen as villains who often partnered up with woozles and selfishly took honey without much thought, which is where a great deal of the fear the Hundred Acre Wood has for them comes from. While there was a rare case of a family of heffalumps who were at least willing to help Pooh, Tigger, and Piglet in a couple of instances in The New Adventures of Winnie the Pooh, after this series ended, heffalumps, as well as woozles, were once again seen as frightening thieves and could not be trusted.
Even though this continued throughout Winnie the Pooh animation in the 90's and early 2000's, Pooh's Heffalump Movie not only reintroduces heffalumps but argues some new ideas about them as a species. This movie introduces a new character named Lumpy, a genuinely sweet heffalump who loves to have fun and make friends. Though initially antagonistic of him, Pooh, Tigger, Rabbit, Piglet, and Eeyore see how harmless he and his mother truly are and ask him for forgiveness. Because of this movie, the new ideas presented about heffalumps are:
Many heffalumps are harmless and show no ill will toward others.
These heffalumps are seen as quadrupeds, regardless of past depictions as bipeds.
Despite past evidence against this, many Heffalumps are more than willing to develop friendships with other creatures.