Personality: Cera's father walks through life with a chip on his shoulder and an unwavering determination to get his point – or points – across as firmly as possible. Hard-headed and bigoted, he never seems averse to putting others down – even going so far as to mess with Chomper's head once in the TV Series (Episode 2: The Mysterious Tooth Crisis). Despite these faults, he does have the redeeming quality of being a very devoted father to Cera, and refuses to let anything stand in the way of his looking after her.

In the later movies and the TV series, he shows a somewhat softer – albeit not always humbler or wiser – side around Tria (see Family), indicating that perhaps she is able to bring out the best in him where others would not think to look.

History: Mr. Threehorn first appears early in the first movie, displaying a range of atitudes from being very attentive and affectionate when Cera hatches, and later when they reunite in th Great Valley, to harshly asserting that "Threehorns never play with Longnecks" just before dragging her away from Littlefoot.

In the later movies, he is often seen taking a very tough, almost Darwinian view of life (as seen particularly in "Stand Tough" from Time of the Great Giving).  Although he tolerates Cera's habit of playing with other dinosaurs he is none too fond of it (Cera quotes him as saying so in The Great Valley Adventure (II)), and generally the first heard speaking in favor of parting ways with other kinds of dinosaurs, as seen in the fifth movie, The Mysterious Island.

(See Family for more)

Struggles: As a single parent for most of the movies, Mr. Threehorn often clashes with his strong-headed daughter, despite the obvious love they share. In The Time of the Great Giving in particular, he temporarily ruined the relationship with a lecture, albeit one which could hardly have been said untrue.

Cera is not the only one whom Mr. Threehorn often irritates; in fact it might be easier to list those he hasn't offended at some time or other. In particular, he often gets into arguments with Grandpa Longneck during meetings among the Valley residents.

Family: Mr. Threehorn seems to be a widower for most of the movies; Cera's Mother only appears a few times at the start of the first movie, and is unseen at the end (it is likely that she perished along the way to the Great Valley). Unsurprisingly, this single father is most often seen interacting with his daughter Cera, whom he does his best to bring up in what he considers the right way. Though the two of them argue rather often, deep down he does manage – most of the time – to hold onto Cera's respect; an achievement not even managed by many of her close friends.

In addition to Cera, Mr. Threehorn has had at least three other children Cera's age (seen in the first movie, implied deceased) and one older child (confirmed to be alive or recently deceased) around the time of Secret of Saurus Rock (VII). Though the last of these was not seen in person, the presence of the twins Dinah and Dana affirms their lineage, as they refer to him as “Grampy Threedee” (“Grandpa Threehorn,” as translated by Ducky), and to Cera as “Auntie Cera.” The fact that they were not seen in subsequent movies implies that at least one of their parents took them back later, though nothing else about their immediate relatives is known.

Never one to let the presence of grandchildren hold him back from anything, Mr. Threehorn later crossed paths with, and subsequently married, an old friend named Tria (who seems to have the singular privilege of calling him “Topsy”). Together they have had one daughter so far, named Tricia, who hatched in Great Day of the Flyers (XII).

Friends: Mr. Threehorn maintains a somewhat rocky relationship with most of the Great Valley residents. He is often seen working in a highly politicized terms with Littlefoot's Grandfather, with whom he openly hates to agree but nonetheless manages to cooperate when the situation demands it. In particular, Grandpa Longneck managed to earn Mr. Threehorn's respet and gratitude for saving his and Cera's lives from a catastrophic fire in The Time of the Great Giving (III).

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Other: Mr. Threehorn is perhaps the only adult present throughout the whole Land Before Time saga to have a given name, although whether “Topsy” is his actual name or a personal nickname is unclear; only his mate Tria uses that name to his face.  In the first movie, the credits list him as "Daddy Topps."