They're called the Sîrênae because of the way you attract them -- you have to sing them or harmonise them into your travel vector during the pre-waking moments of the last segment of sleep; when one's technically awake but still in firm contact with the CCRB.
They're like fish you catch. Space Whales are the largest and most phenomenal of the species. Space Sharks are the most agile yet fearsome. Space Spiders, now there's a real horror, but the most versatile. Space Worms are soft as kittens. Space Starfish are amazing help.
The art of contacting the Sîrênae (as I've called them) is to sing in one's pre-waking lucid dreams but all will be explained as DDA1 or Dream Detective Agency (book one) unfolds.
FURTHER CONCEPTUAL UPDATE: it's fun to go aquatic on this, but what I'll end up doing is go Astrological on this. Capricorn-fish and Taurus-bulls... Glimpsers are merely the horns of the Taurus-bull. The Kîmêrae are the legendary Dreamers who can ride these ancient Sîrênae beasts across space and across the ages. They're a Voyager race, with stolen secrets to keep.