The children stood round the hole in a ring, looking at the creature they had found. It was worth looking at. Its eyes were on long horns like a snail's eyes, and it could move them in and out like telescopes; it had ears like a bat's ears, and its tubby body was shaped like a spider's and covered with thick soft fur; its legs and arms were furry too, and it had hands and feet like a monkey's...
The Sand-fairy smoothed his long rat-like whiskers and smiled between them.
I suspect that this description is so full of specifics that it ceases to be fun for the reader to imagine and starts to become obstructive to the narrative. If you have to try and remember all that every time the Psammead pops up in the story, it is hard to concentrate on anything else! This is a nice opportunity for an illustrator to help out... but of course the illustrator has to make sure they do give their Psammead ALL of the features described. You know some readers are going to be checking! That is exactly the sort of thing I used to do when I was a child ^-^
Even a close description like that does allow lots of room for interpretation though:
What would your Psammead look like?