This took a while, but I've got it.
As far as I know, there are no venomous animals on the island. However, the question states "What venomous animals can be found in Ibiza?" The word 'in', being more general, would appear to mean in the genral vicinity of the island.
There is a type of jellyfish (or medusa) called the Mauve Stinger (Pelagia noctiluca) that populates the waters around the island. According to the site Toxinology.com, the mauve stinger is: "Large jellyfish, sometimes found in swarms, causing painful stings with wheals, pruritis and swelling. Appears to have allergenic venom. There is no antivenom."