In the various versions of the Mysterious Island movies or mini series, they have giant animals. Are they in Jules Verne's original story or were they added by Hollywood?
Little of yes, little of no. Verne's Mysterious Island (which you can read online here) repeatedly makes mention of unusually large creatures (a dugong, a whale, moufflons, an ape-man creature) as being a feature of the island.
The sea monsters and giant lizards, however, are purely an invention of Hollywood.
Although not a manatee, it was a dugong, which belongs to the same species.
The huge monster threw himself upon the dog. His master could do nothing to save him, and, before Spilett or Herbert could draw their bows, Top, seized by the dugong, had disappeared under the water.
Neb, spear in hand, would have sprung to the rescue of the dog, and attacked the formidable monster in its own element, had he not been held back by his master.
Herbert and Spilett, who were about starting on a hunt, laid aside their guns, Pencroff dropped his hatchet, and Smith and Neb, joining their companions, hurried down to the shore. It had grounded on Jetsam Point at high water, and it was not likely that the monster would be able to get off easily; but they must hasten in order to cut off its retreat if necessary. So seizing some picks and spears they ran across the bridge, down the Mercy and along the shore, and in less than twenty minutes the party were beside the huge animal, above whom myriads of birds were already hovering.
“What a monster!” exclaimed Neb.
And the term was proper, as it was one of the largest of the southern whales, measuring forty-five feet in length and weighing not less than 150,000 pounds.
They had need to hasten, for at a turn in the path they saw the lad prostrate beneath a savage, or perhaps a gigantic ape, who was throttling him.
To throw themselves on this monster and pinion him to the ground, dragging Herbert away, was the work of a moment. The sailor had herculean strength. Spilett, too, was muscular, and, in spite of the resistance of the monster, it was bound so that it could not move.
The colonists descended but slowly. They experienced some emotion in thus adventuring into the depths of the earth, in being its first human visitants. No one spoke, but each was busied with his own reflections and the thought occurred to more than one, that perhaps some pulp or other gigantic cephalopod might inhabit the interior cavities which communicated with the sea. It was, therefore, necessary to advance cautiously.
The corral finished, the next thing was to inaugurate a grand hunt at the pasturages, near the foot of Mount Franklin, frequented by the animals. The time chosen was the 7th of February, a lovely summer day, and everybody took part in the affair. The two onagers, already pretty well trained, were mounted by Spilett and Herbert and did excellent service. The plan was to drive together the moufflons and goats by gradually narrowing the circle of the chase around them. Smith, Pencroff, Neb, and Jup posted themselves in different parts of the wood, while the two horsemen and Top scoured the country for half a mile around the corral. The moufflons were very numerous in this neighborhood. These handsome animals were as large as deer, with larger horns than those of rams, and a greyish-colored wool, mingled with long hair, like argali.