"Vulcan children are never late with their sehlat's dinner."
- - T'Pol
The sehlat was a giant "teddy bear"-like beast, native to Vulcan. Sehlats had six-inch fangs and did not like to climb, preferring to remain on low ground. In 2154, Captain Jonathan Archer and Commander T'Pol were stalked by a wild sehlat in Vulcan's Forge.
Domesticated sehlats, which were only slightly smaller than wild sehlats, were a popular pet with Vulcan children. Sehlats, however, were very aggressive, so Vulcan children learn early never to be late with their dinner. T'Pol had a pet sehlat in her youth. (ENT: "The Forge")
Syrran and T'Pau were able to imitate the screams of a sehlat, and the latter used imitations of those noises to mislead Talok and two Vulcan commandos into the electric discharges of gallicite deposits. (ENT: "Kir'Shara")
Spock had a pet sehlat named I-Chaya during his youth that originally belonged to his father, Sarek. I-Chaya died in 2237, as a result of injuries inflicted from an attack by a le-matya. Because the venom of the le-matya was incurable by that point, Spock chose to have I-Chaya euthanized to end his suffering. (TOS: "Journey to Babel"; TAS: "Yesteryear")
Background information Edit
Although the sehlat featured in "Yesteryear" was domesticated, the potentially harmful nature of sehlats in general is referenced in the episode's script. Writer D.C. Fontana explained, "My written description of a sehlat in the 'Yesteryear' script includes something to this effect: 'Make one wrong move, and a sehlat will rip your arm off. However, Vulcans don't make wrong moves. That would be illogical.'" (Babel #5; Enterprise Incidents, number 11, pp. 27 & 29)
According to the unauthorized reference book Boldly Writing (p. 5), the animated design of the sehlat was inspired by a speculative article that was published in the 1970 fanzine "Spockanalia 5". Boldly Writing states, "In this piece, the author looks at precedents in nature to see what sort of animal would have six-inch fangs. The author concludes, 'And so our portrait of the sehlat: a carnivore or just possibly tusked omnivore, general shape that of a giant panda, size on the order of an Alaskan brown bear, highly intelligent, and despite the six-inch fangs, of a patient and gentle disposition.... Question: did the sehlat belong to Spock, or was Spock in the care of the sehlat?' The Star Trek production staff read this article, and animators incorporated many of the suggestions into the drawing of the sehlat I-Chaya." 
Another basis for the animated appearance of the sehlat was an illustration that fantasy and science fiction artist Alicia Austin drew for D.C. Fontana, who remembered, "Alicia Austin came up with several variations on what a sehlat looks like. I chose one of the early ones as a model – with modifications [....] I-Chaya has a different kind of tail, different shape of face, the broken fang." (Babel #5; Enterprise Incidents, number 11, p. 27) Fontana purposefully instructed the Filmation artists to break off I-Chaya's tooth, in order to give the sehlat a more aged appearance. 
The sehlat seen in "The Forge" was a CGI character. With indirect references to the sehlat, Dan Curry recalled how he and a visual effects team at Eden FX revamped the sehlat for Star Trek: Enterprise; "We looked at the animated series and it just looked nice and pleasant, so I did a couple of sketches to do a reinterpretation of it to make it look scary, but not be too radical a departure from the original [....] John Teska at Eden FX modeled it and and visual effects supervisor Art Codron supervised the compositing. We used special software to create the fur and we did it so should the need arise we could look at it fairly closely." (Cinefantastique, Vol. 37, No. 2, p. 37)
In one scene of the Enterprise episode "Terra Prime", T'Pol was originally to have sung to her daughter, Elizabeth, a Vulcan lullaby about why children are never late with their sehlat's dinner, though this idea was scrapped. ("Terra Prime" audio commentary, ENT Season 4 DVD special features)
The text commentary for "Yesteryear" on the Star Trek: The Animated Series DVDs, which was written by Michael and Denise Okuda, claims that Tuvok also had a pet sehlat. However, no canonical evidence confirming this from Star Trek: Voyager is immediately evident.
The Okudas' text commentary for Star Trek: Enterprise's pilot episode, "Broken Bow", on the ENT Season 1 DVD supposes that sehlats typically smell less foul than dogs, since several Vulcans (naturally hypersensitive) are known to have had a pet sehlat whereas T'Pol found Captain Archer's pet beagle, Porthos, to have a particularly bad odor.
In the Pocket Star Trek: Enterprise relaunch novel Kobayashi Maru, the corpse of a sehlat was found on the floor of a darkened cave on the Vulcan outpost planet Trilan by T'Pol in 2135 (Vulcan Year 8737), the species having been taken to the outpost planet by its Vulcan settlers. T'Pol took bones from the seh'lat's skeleton and later used them to defend herself against a group of Fri'slen, brutal and primitive humanoids who were actually mutated Vulcans. Sehlats are described in the book as an "urso-feline species" and sharp tusks are said to be characteristic of "adolescent-to-fully-grown members" of the species.
The novelization of "Yesteryear" (in Star Trek Log 1) states that sehlats originated in the rain forests of Vulcan's southern hemisphere. The same adaptation also refers to I-Chaya's brown coat as looking "faded in spots to patches of pale beige," and his fangs as being "yellowed" and ten centimeters long, although these physical characteristics are not in keeping with the sehlat's appearance in the episode.
In Star Trek Online, players can have a pet sehlat cub. They are also more feline than depicted in main canon, resembling sabertoothed Lions.