The Kraan are one of the first monsters encountered in the Lone Wolf series.
They are described as leather-winged flying creatures and in game terms as having a combat skill of 16 and an endurance of 25. A very powerful foe right out of the gate.
One of the main problems with a conversion between the AD&D system and the LW gamebooks is the skill-based system and relative uniformity of the monster strengths as compared with the level experience system and incremental increase in strength and power of both characters and monsters in the AD&D system.
The Kraan are perfect example. If they are to be encountered as monsters to be fought directly from the very first then they must be represented as creatures of relatively low HD. For a game beginning with 1st level PCs then it is my belief a Kraan of no more than 2HD should be encountered. A party of 4 1st level PCs will be challenged to defeat a 2HD Kraan with 2 attacks bite 1d4/ claw 1d6 AC7 but completely overwhelmed by a more powerful, larger version that can act as a mount for enemy NPCs.
For my game conversion I'm using Kraan of varying ages. The youngest encountered are Spawn. They are 1HD flying creature with a single slashing attack of 1-2HP damage. They run to about 50lbs and are AC8. They are normally encountered in groups of 1d6+1
2-3HD Kraan are Young and are double the size of the Spawn. They are still too small to act as mounts and more solitary. 1d4 are the normal number encountered.
Young Adult Kraan are 4-5HD, AC5 with 3 attacks Bite 1d6 and Claw/Claw 1d8/1d8 and are more than twice the size of Kraan Young. These creatures are much larger than humans and are trained to be mounts, but are still not used as mounts for the Darklords but smaller Humans and Giaks will be found riding them.