Thlayli (the name is Lapine, meaning fur-head in English) is informally known as Bigwig. He was born in the Sandleford Warren and was given the name Thlayli because of a bushy patch of fur on top of his head. After his second year, he became of an officer in the Sandleford Owsla under Threar-rah.
During his tenure in Sandleford, two rabbits named Fiver and Hazel came to be known throughout the warren; Fiver for being a gifted seer, and Hazel for being his older brother and guardian. Although Fiver's premonitions were often proven to be correct, he was turned away from Threar-rah's burrow after suggesting that all of the Sandleford rabbits relocate in the face of certain doom. Thlayli heard Fiver's advice to Threar-rah and in fact was the one who allowed them to see each other. He was an acquaintance of Hazel's and knew of the accuracy of Fiver's sight. When he heard that Fiver had seen the entire warren destroyed, he grew worried, and decided to follow Hazel, Fiver, and a group of other rabbits who had decided to break away from Sandleford.
The group traveled through forests and fields. After losing a rabbit to a hawk, they arrived at a strange warren dominated by a simpering rabbit named Cowslip. Cowslip readily offered his hospitality to the Sandleford rabbits, including a pile of flayrah---rare and delicious food. Fiver was suspicious of the situation. For him, it was too good to be true. Thlayli, however, was hungry and exhausted from their journey and lost his temper with Fiver, running off to join the feast. He was almost immediately caught in a shining wire, a man's trap. It became apparent that a man was providing Cowslip's warren with food. In exchange, they had forsaken the old traditions and did not try to escape the man's traps.
Thlayli was strangled almost to death by the wire before his companions dug out its peg and set him free. For a few minutes, he was unresponsive, but he woke during the eulogy being said for him. As he stood, bleeding from the nose and mouth, he vowed to kill every rabbit in Cowslip's warren. It was Fiver who convinced him to simply come away and continue with the journey. After this incident, Thlayli accepted Fiver's advice almost without question.
Sometime later, after passing through more farmland, the rabbits came across Thlayli's former captain. Captain Holly was the only survivor of the disaster that befell Sandleford: extermination by humans. He also survived imprisonment in the warren Efrafa. The group stayed with Holly until he recovered, then took him with them to the top of Watership Down. There, they founded their warren and Hazel was recognized as chief with Thlayli as his captain---Holly, due to his traumatic experiences, was no longer fit for the job.
While living at the down, Thlayli befriended a seagull named Kehaar. Kehaar told the warren more details about Efrafa, an overcrowded warren whose females were trying to break away. Despite Efrafa's viciously territorial nature, the Watership Warren was desperate for does, and Hazel decided to approach General Woundwort to see if he would relinquish some.
On the way to see the General, Hazel's emissary group encountered a fox. Thlayli lured it away and straight into an Efrafan patrol that had been stalking the Watership rabbits. The fox killed the patrol's captain, and the patrol arrested Thlayli without discovering his friends. However, when Woundwort heard of what Thlayli had done, and then saw him in person, he made him an officer. This granted Thlayli considerable power within Efrafa and allowed him more freedom.
While in Efrafa, Thlayli worked to bring as many does as possible back to Watership Down. He relayed messages home via Kehaar. His successful escape enraged General Woundwort, who swore to kill him personally. Soon, the General discovered the Watership Warren and attacked it with the plan to kill every rabbit there. Hazel escaped with several other rabbits to release a dog from a nearby farm in order to lead it to attack the Efrafans. He left instructions with Thlayli to hold the warren until they returned. Woundwort ignored their escape because he was under the impression that Thlayli was chief.
Thlayli ambushed Woundwort from beneath his feet and fought him until they were both exhausted. At that point, Woundwort asked Thlayli why he refused to surrender. Thlayli declared, "My chief's told me to defend this run," a statement which surprised and intimidated Woundwort. At that point, the dog arrived at the warren and began massacring the Efrafans, including Woundwort when he went outside. Thlayli was severely injured during the fight, but survived to continue on as Watership's scarred and battle-wizened Captain of the Owsla.
The darkness came a few months after peace was finally granted to the Watership rabbits. Shadows began to infiltrate the land and now the flay is no longer nutritious. Does no longer give birth and bucks rarely reach adulthood. Rabbits are scrawny and thin and unknown enemies stalk the perpetual night. Hazel has once again tasked Thlayli with an important mission: find a safe place once again. Thlayli has not yet passed through the portal, but he has begun to hear stories and is on the right track. If all the rabbits could become men, he thinks, things would be much better.