The bloodthorns can double hit for up to 93 points. They are definately the toughest 'common' mob in the Emerald Jungle, especially since they blend in quite nicely to the terrain.
Also, they *do not* see through invis. I was attacked by one while sitting next to my shadowknight friend. She had Gathered Shadows (was invis), and the bloodthorn attacked me. By innstinct I feigned death, and the bloodthorn sedately plopped away, ignoring my invis friend right next to me.
Of course, it's possible that it was a lower level version of the bloodthorn, and the higher level ones CAN see through invis. Methinks the best advice is just to avoid these guys unless you're very confident in your group. :)
The "Blood Thorn Extract" which drops off these are actually used to make an injected poison, named Inferno Blood; it's not a contact poison. Although it is a very powerful poison...one of the strongest actually.