Originally, in the comics, Venom took that form after bonded with Eddie purely as a distortion of the look Spider-Man has, taking the "perfect host"'s appearance and making it more dangerous and intimidating due to the shared hatred the symbiote and Eddie shared for Peter. Since then, the other symbiote designs we've seen have mostly been direct offspring of Venom, ie Carnage, Riot, etc., which explains why they look like Spider-Man with sharp teeth (copying Venom's look due to being direct descendants). The standalone movie is the only piece of media I can think of that has Venom and the other symbiotes look like that without any link to Spider-Man, and outside of that film, they probably take on whatever appearance most strongly resonates with their host, without the distortion of the teeth and tongue and such.