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Honestly, I'm not even sure the wildlife of Ygam is all that strange or fantastical. The trouble the Oms face when left to scavenge for themselves outside the cities is all down to their being the size of insects relative to their surroundings. Looked on from the Draags' scale, the fauna mostly fall into functions we'd recognize on Earth - sedentary feeders who dress their chosen spot all appealing and then strike at prey, scavengers what feed on the young of other species, avian-like strike-and-kill hunters, the works. 322
Their morphology is exceptionally distressing to our alien eyes, and those foaming snail-like creatures which willingly spin clothing for the Oms defy conventional classification, but through a strictly xenobiological eye, there's little reason to take surprise at the prevalence of physical traits not commonly found in our home environs, nor the possibility of domestication among different species classes. It's all easily overcome through the methods the Oms adopt thanks to Terr's theft from the Draags anyhow, attentiveness to and application of knowledge about the environment and its systems. Survival against anything less than overwhelming, ideologically determined forces is a simple matter of understanding, level-headedness, and more than a little luck, so the sooner one's over the shock of the humanoid face and inscrutable behavior of this lad, the better their odds.
Now, the spontaneous generation of solid crystals in much the way we experience morning dew? The Draags' form-altering meditation sessions in which they become little more than blobby, warping nervous systems with heads on? The seemingly natural occurrence of long-distance psychic receptors on Ygam's satellite whereupon the Draags commune with alien beings? THAT stuff all goes beyond conventional scientific understanding and deserves the name "fantastic."
(I'd draw a similar line about the Draags' inability to conclude the Oms are intelligent and civilized same as them despite the common body layout, the capacity for language skills, the organized nature of their homes and movement patterns, and the clear signs of social inhabitance on their home world... but The Parallels between the high-minded alien species' failure to recognize kinship with those they automatically deem lesser and Certain Real World Problems Prevalent In The Filmmakers' Native Europe is rather The Point of the movie, so it oughtn't be lumped with the more jesting aspect of this piece.) 802