The Grell are mud bathing, cackling, bird-brainapuses that consistently say things like "Duuude!" and "Woah" in between their other frankly inarticulate ramblings. If you were to list every Fiend Folio monster's most annoying attribute and ranked them, the Grell would have at least two spots in the top five. The only intelligent thing they seem to do, which has obviously evolved instinctively, is mud bathing because much like the pig they don't have sweat glands and a big brain needs to keep cool. If you'd like to meet one of these guys bring a sixer of brewski's to the closest mud pit and hoot, holler and shout like a total idiot until one shows up to join the party.
These Grell eat and disgustingly enjoy tadpoles. Frogs somehow completely disgust the Grell but the frog babies are like the shallot cream cheese on an everything bagel: it just doesn't make sense that something so simple could be so delicious. I have no speculation to why these partially-formed creatures are so delicious, really. They aren't adverse to eating worms and other slimey-wimey gross things for subsistence's sake but prefer "and irritatingly only talk about" eating tadpoles.
The one possibly-redeeming quality of these creatures is that their ladyfolk are equally as shameful and ignorant as the men. "Here, here; for monster gender equality!" Their mating practices take tentacle porn to an exciting new level; just imagine a giant brain being fingered (or tentacled) by a bunch of sticky but slimy purple tentacles. The sounds they make during coitus are possibly the most horrifying thing about them. Listen to the mixing of bubble wrap popping and wet, blood curdling bird of prey screeches and you might get an inkling of the messed up, terrifyingly erotic sound.
After Consulting the Text
These guys have a beak, 5' diameter brain lobes, and 10 6' tentacles wiggling around under them which lend to their freaky hovering ability. The most likely place to run into one of these ghastly creatures is underground or in abandoned buildings, floating near the ceiling waiting for you. Sadly these ugly beasts can only really attack one opponent at a time, wrapping their tentacles around them and injecting their prey with paralyzing venom. The kickass part of their attack is when its prey rolls satisfactorily on it's saving throw, check it; the Grell still has 2 tentacles attached to you! What?! You will be very lucky to get away alive. Plus the only way to kill a Grell is by hitting it's massive brain body, hitting a tentacle does nothing but make it inoperative.
On an end note: grell totally sounds like a curse word. As in, "you grelling toad." Or that could just be the Farscape talking. The End.