From Lexicanum

The Catachan Devil is a creature native to the Death World Catachan, but has been introduced into several other worlds. They are scorpion-like in appearance with a series of segmented body sections and pairs of legs terminating in a long tail with a poisonous barb, which curves forward over the creature's back. As Catachan Devils grow they develop more and more mid-sections and pairs of legs. A fully grown adult can have up to thirty sets of legs, reaching up to thirty metres in length, reaching the size of a land train.³

The front legs of older male Devils become grossly enlarged, resembling scorpion claws. These mighty beasts are often dubbed 'Fiddlers' in Catachan slang, and are quite rightly feared for it's ferocity and raw power.

They are able to move with suprising speed for something of it's size, being superbly adapted to move across a large variety of terrain. Once capturing it's unfortunate victim, it will either shred it with razor claws or stung into insensibility by barbs around the maw, before finally being consumed.

Catachan Devils are territorial, yet quite socialable creatures, tending to form into nesting groups. These groups often contain several large Devils, as well as a few dozen or so of 'Devilspawn'. The entire nest will come together to defend their territory from any intruders, be it rival Devils, colonists, other predators, and have even been known to attack Imperial Guard Tank Companies.⁴

It is also rumoured that they are the descendants of a long-lost Tyranid Vanguard organism that has effectively become feral.

Sources

• Warhammer 40,000: Rogue Trader
• http://uk.games-workshop.com/warhammer40000/creature-feature/3/
• ³ Codex: Tyranids (4th Edition)
• ⁴ White Dwarf Issue 292 (au) - Chapter Approved, Creature Feature, Pg.35