The two main reasons for a morph being rendered unusable is either the morpher becoming a nothlit (trapped in one morph), thus leaving him or her unable to morph out, or because the individual acquired the morph in a Sario Rip. A Sario Rip is a rift in the fabric of space-time, caused by a massive, precise explosion. It can send those in the vicinity of the explosion either forward or backwards through time. If a person manages to escape the effects of the Sario Rip and prevent the existence of the timeline in which the morphs were acquired, they would lose their ability to morph into those animals.
There are also other possible reasons: those suffering from a hereth illint, an allergic reaction to an animal's DNA, would expel said DNA, rendering the morph unusable. In a rare instance, one could also acquire and morph a creature that goes through natural morphing (such as a caterpillar morphing into a butterfly), thereby rendering both forms unusable.
|Rachel||Spider Monkey (Male), Jaguar, Crocodile, Deinonychus, Crayak's Monsters|
|Tobias||Cat (Dude), Deinonychus|
|Cassie||Spider Monkey (Male), Jaguar, Leeran, Tyrannosaurus Rex, Caterpillar/Butterfly|
|Marco||Spider Monkey (Male), Jaguar, Leeran, Tyrannosaurus Rex, Grizzly Bear|
|Ax||Spider Monkey (female), Jaguar, Leeran, Tyrannosaurus Rex|
|Esplin 9466||Amazonian Snake, any morphs acquired through host body Alloran-Semitur-Corrass|
|David||Golden Eagle, Cockroach, Flea, Lion, Seagull, Human (adult male Controller), Human (Saddler), Human (Marco), Rattlesnake, White Rat|
|Arbron||Kafit bird, Djabala|
|Elfangor-Sirinial-Shamtul||Kafit bird, Djabala, Taxxon, Human|