These ‘dinosaurs’ are either junior synonyms, or misidentifications of other dinosaur remains.
"Cetiosaurus brevis"
An alleged sauropod, known on the Isle of Wight from several vertebrae from Sandown Bay and Culver Cliff and very fragmentary vertebrae and limb elements from the Wessex Formation of Brook. If all these belonged to the same individual, then they would have been the second most complete Isle of Wight sauropod, diagnosed on the basis of a low centrum length: height (l:h) ratio, a derived condition present in the anterior caudal centra of eusauropods.
However, it has been demonstrated that all the material assigned to C. brevis by Owen belongs to Iguanodon (which also has a low l:h ratio in the posterior dorsals), with the exception of BMNH R2544–2550 which represent an indeterminate sauropod. Most specimens are too poorly preserved to be identified precisely and may not even be confidently assigned to the sauropoda.
Dollodon
During the splitting of Iguanodon into multiple genera, (including the still valid Mantellisaurus), two species of Dollodon were erected to house remains from Iguanodon seelyi and Iguanodon bampingi. More recent research has confirmed that these species are actually examples of Iguanodon bernissartensis and Mantellisaurus atherfieldensis.
"Megalosaurus"
In the early days of palaeontology, Megalosaurus was something of a ‘waste basket’ taxa, into which pretty much any new dinosaur material that wasn’t clearly Iguanodon or Cetiosaurus was put into.
If it’s on the Isle of Wight, then it’s most likely Neovenator.
Polacanthoides
Known from an isolated tibia, humerus and possibly a scapula, almost certainly Polacanthus
stegosauria indet (Regnosaurus?)
Known only from a single incomplete pubis, with a stout prepubic process and a slender postpubic process, similar to Iguanodon. The prepubic process shows no flaring and there is no anterior recess next to the position of the acetabulum, which originally suggested it belonged to a stegosaur.
However, the acetabular surface of “Regnosaurus” laterally, posteriorly and dorsally, in stegosaurs this surface faces wholly laterally. It is therefore more likely that this specimen is from an Iguanodontoid.
Vectensia
nomen nudem, known only from a triangular flat sided bony spike, about 25cm (10 inches) long, but possibly Polacanthus
Vectisaurus
Juvenile specimen of Mantellisaurus