Nate is a 12-year-old boy who lives on a small farm while his father runs the local paper. He enjoys a slow-paced life filled with chores and fishing at Loon Lake. When one of the family hens lays an abnormally large egg, Nate takes on the responsibility of carefully turning and monitoring it. He is patient, dedicated to animal welfare, and fiercely loyal to his unconventional new pet.

Walter runs the local newspaper, the Freedom Sentinel, and supplements his income with a small farm. He approaches life in his small town with practical pride and supports Nate's curiosity. Though he initially warns his son to prepare for disappointment, he secretly visits the henhouse at night to turn the egg himself.

Dr. Ziemer is a paleozoologist from Washington, DC, spending his summer vacation in Freedom. He identifies the strange egg's scientific potential and encourages Nate to keep waiting for it to hatch. Open-minded and enthusiastic, he mentors Nate through the complexities of raising a dinosaur and managing the scientific community's sudden interest.

Uncle Beazley is a Triceratops who hatches from a hen's egg in the Twitchells' backyard. Named after Mom Twitchell's great-uncle, the dinosaur possesses an enormous appetite for grass and leaves. He grows at an astonishing rate, transitioning from a small yard pet to a massive creature that requires constant attention.

Senator Granderson is a bald, cigar-smoking politician who takes an interest in the dinosaur's presence in Washington. He views the Triceratops as an unnatural expense to the taxpayer and uses the situation to propose restrictive legislation to further his own political goals.

Nate's mother manages the household and values routine and order above all else. She prefers a quiet life and worries about the disruption the massive egg might bring to their schedule. When scientists and reporters begin arriving, she expresses frustration at having to delay supper and accommodate the crowds.

Cynthia is Nate's younger sister. She spends her summer helping her mother in the kitchen and occasionally takes over Nate's chores, such as milking the goat. When the dinosaur hatches, she initially acts squeamish but soon volunteers to answer the telephone as the family's acting secretary.

Dr. Kennedy works at the United States National Museum in Washington, DC. When summoned to New Hampshire, he arrives frazzled and thoroughly skeptical of the dinosaur claim. He represents the established scientific community's cautious approach to impossible discoveries.

Joe is Nate's best friend and frequent fishing companion. Influenced by his father's distrust of scientists, Joe initially insists that the newly hatched creature is just a large lizard. He eventually helps Nate mow overgrown lawns to secure food for the dinosaur.

Mrs. Parsons is the Twitchells' next-door neighbor in Freedom. She takes great pride in her flower beds and prefers a quiet neighborhood. Her early complaints about the family rooster establish her as someone who keeps a close eye on the Twitchells' property.

Ezekiel is the Twitchell family's rooster. His loud morning calls nearly get him evicted from the farm until Nate formulates a plan to keep him in the cellar overnight.

Mrs. Beebe is the local telephone operator in Freedom. She is familiar with the town's residents and their routines, initially hesitating to connect Nate's early morning call. She later serves as a crucial filter when reporters and scientists flood the Twitchells' phone line.

Professor Morrison is a leading paleontologist from Montreal. He travels to New Hampshire to see the Triceratops and offers Nate advice about protecting the animal from commercial exploitation.

Bill Griner is a businessman who visits Freedom hoping to purchase the dinosaur. He wants to use the animal as a roadside attraction to draw customers to his gas station, representing the commercial interests eager to profit off the scientific discovery.

Henry Smith is a local resident of Freedom who owns a tractor mower. When the dinosaur's appetite outgrows Nate and Joe's ability to cut grass by hand, Henry uses his equipment to harvest fresh grass from the side roads.

Michael Finney is a truck driver employed by the National Museum. He helps transport the massive Triceratops, preferring live animals over the fossilized bones he previously dug up during an expedition in Wyoming.

Holmquist is the director of the National Zoo in Washington, DC. Having recently lost an elephant, he has the space required to house a massive creature and is eager to accept the dinosaur despite severe federal budget constraints.

Bonelli runs a television program called Capital Sidelight. He invites Nate onto his show to discuss the dinosaur, though he attempts to strictly control what the boy says on the air to fit his broadcast format.