Slugbulbs are small glowing dragons with a toothed mouth, a tail, an a round body. They appear to have filaments or tentacles of some sort. Some depictions show small knobs. They have one pair of wings. They also don't appear to have any eyes in their images.
Slugbulbs appear to be able to glow or fluoresce based on their pictures and name.
Hiccup and crew encounter them in the caves beneath Flashburn's School of Swordfighting in How to Steal a Dragon's Sword.
An image of a slugbulb appears in the chapter concerning Cave Dragons.