I have been a fan of darkenbeasts since I first discovered them in 2E (I think from reading the novel Red Magic which was one of the books in the Harpers series) but I have never used them. Of course, if I am going to run a campaign involving Thayans I need to fix that oversight so I thought I should make some notes here.
Right now I am thinking of them in the context of my plans for what was Dark Weavings but now is The Black Age of Bane again. Before explaining why, I should note exactly what a darkenbeast is (and, for one thing, it is a lot darker than the picture above - from 3E's Monsters of Faerun - would indicate). A darkenbeast is simply a normal animal transformed by a Thayan spell (pre-4E) or ritual (4E) into a flying beast that resembles a cross between a wyvern and pterodactyl. They can only stay in darkenbeast form in areas of darkness. Sunlight causes them to revert to their natural form.
Right now I am imagining an encounter which begins with the PCs discovering a pair of horses that have died apparently from falling from a great height. A little further along the PCs discover the remains of a merchant caravan that was apparently torn apart by wild beasts. Of course, the two scenes are linked. The horses were drawing one of the wagons before being transformed by Thayan wizards and let loose on the merchants. When the sun arose the next morning, the horse-darkenbeasts were still in the air when they were transformed back into obviously non-flying horses. When the Companion is corrupted in the final adventure of the Heroic Tier, perhaps every normal animal that its darkness touches is transformed into a darkenbeast... but what would be, in 3.xE terms, a darkenbeast beast of Bane. Something like this:
I also imagine that they will be fairly common encounters through the various Dread Ring fortresses/dungeons. Mechanically, I want to stick with the 5 hit dice they had in 2E and make them 5th-level skirmishers and essentially fairly boring. The most interesting thing about them is their link to the wizard that created them (that and their reversion to their normal form in sunlight).