The Lackwit is a dragon mentioned only on the How to Train Your Dragon Books official website as Viking initiate, Clueless' Hunting Dragon.
Physical Description
No description or image is given for the Lackwit. It is, however, mentioned to be a Hunting Dragon, implying that it is not big enough to ride and probably the size of a dog.
Behavior
Lackwits are described as being "very violent and unfocused"[1].