Final answer:
The direct object in the sentence is "two very frightened rabbits".
Step-by-step explanation:
The direct object in the sentence "A dinosaur called Donnellsaurus caught two very frightened rabbits by their tails" is "two very frightened rabbits"
The direct object is the noun or noun phrase that receives the action of the verb. In this sentence, the action of the verb "caught" is performed on the rabbits.
The direct object can usually be identified by asking 'what?' or 'whom?' after the verb. In this case, we can ask 'caught what?' and the answer is 'two very frightened rabbits'.