Source: City in the Deep, pg(s). 87
The tullimonstrum is a blood-drinking, deep-sea fish that displays features of invertebrates and can serve spellcasters as familiars. While tullimonstrums are relatively weak individually, a school poses a lethal threat to nearly any target.
A tullimonstrum is a small cylindrical creature with two eyestalks extending straight off to each side. A typical tullimonstrum is 14 inches long and weighs six pounds.
- Wikipedia:Tullimonstrum (real-world animal)