A velium crawler

Level 30 · sea dragon · ? · Crystal Caverns

These little monstrosities, known for inconveniencing travelers in Crystal Caverns, are an excellent source of experience through level 40, especially if you have enduring breath and can venture into the frozen river beneath the zone to fish them out. Most monsters in CC have a wide level range, but velium crawlers are always exactly level 30. Some see invis (especially the ones in the pool at the geo/spider/terror 4 way).