Creating imaginary creatures

A fantasy setting is usually closer to the world as we know it than a science fiction one. It’s more accessible, and consequently more attractive to a child who is still awed by things that are all too familiar to an adult. Fantasy is unlikely to change gravity, or body chemistry, or a breathable atmosphere, so the creatures that populate that world would be at home in ours. This is why many mythical beasts are clear mixtures of two or more real animals – the griffin is half eagle and half lion, and the minotaur half bull, half man.
When I needed to invent a really scary animal myself, I fell back on a real life experience. I was camping in a tiny tent in Tsavo East National Park. At two in the morning I was woken by a clattering sound – something was going through the dirty dishes – and, horror of horrors, I needed the loo, which was on the other side of the clearing. The scavenger could have been practically anything; a leopard, a lion, a baboon… The clattering eventually stopped, and just as I was plucking up courage to unzip the tent, something sniffed – right on the other side of the canvas. It had to be something big, as the sniffing was waist-height. I lay there, absolutely petrified, until dawn. When I finally ventured out of the tent there were the paw-prints of a hyena all round the outside.
Hyenas are pretty weird creatures anyway – exposed to huge levels of testosterone whilst still in the womb, they are so aggressive that in a litter of two the first born will often attack and kill the second born within minutes. Their jaws can crunch bones, and their calls sound uncannily like laughter. I added a dash of shape-shifting, so that they could disguise themselves as something else, intelligence so that they could plan their murder and mayhem, a disgusting smell – and a voice with which to properly express their gruesome intentions. In The Divide, our hero, Felix, first encounters them in the forest with his elf-friend Betony.

…Felix saw something move. Something a bit like a dog, but not quite. Something front-heavy, with ears that looked a little too big for it. Something with spots.
“So,” said Architrex, emerging from the shadows in his hyena-shape, “this is a human child, is it?”
A faint smell wafted across the glade; not a very pleasant one.
“You’re a sinistrom,” said Felix, feeling rather pleased with himself at the snap identification. But when he looked at Betony, she’d gone as white as a sheet and her hands were gripping the rucksack as though she wanted to squeeze the life out of it.
“Very good, human,” said Architrex.
Vomidor stepped out of the shadows next to him. “Do you want me to dispose of the tangle-child straightaway?” he asked Architrex.
“What do you mean?” said Felix.
“We don’t need Betony,” said Architrex. “We only need you.”

I would define a fantasy creature as one that has been invented by the author to serve the story he or she is telling. How believable do you think it ought to be?