Ellieandrose, the lizard is called the Kanima. The kanima was revealed to be Jackson, after he recieved a bite from Derek Hale sometime during the first season. Jackson's body, as far as we know, had different effects and did not turn him into a werewolf like he wanted, but instead turning him into the Kanima. So far, the characters have found out that Jackson is being "controlled" by an unknown person. With the latest episode, the person controlling Jackson is believed to be Mr. Harris, a teacher at Deacon Hills High School.
If you google Teen Wolf Wiki, you'll find everything you need to know catch up.
And super good news time! They've renewed for a third season!