Well, when the Venom symbiote takes over a new host, it generally just looks like Venom. There two notable exceptions were in Marvel Knights, where the symbiote took on a new design after taking a new host. However, when that host died (the symbiote left him to die, deciding he was too weak to serve as a proper host), the symbiote kept the new look when it then offered itself to Mac Gargan, formerly the Scorpion.
As seen here:
The other exception to the "They still look like Venom" rule is the short lived female Venom, alternatively called She-Venom or the Bride of Venom. She was formed when Eddie Brock, desperate to save his ex-wife from critical injuries, forced the symbiote to join with her and heal her. They joined again at least one more time I'm aware of. Sadly, the process of joining with the symbiote drove the woman insane, and she killed herself later.
Still, the female Venom still held to the traditional Venom design, except...well, you can see for yourself.
Hope that answers your question.