It's a fish! It's a dolphin! Wait a tick - it's neither of those things. Just what the hell is it?!
That, friends, is an Ichthyosaurus. That name literally means "fish lizard." It lived in the Jurassic - about the time that Allosaurus was the dominant carnivore on land. The seas were also populated by lizards during the Mesozoic, and Ichthyosaurus was among them. It ate fish (well, yeah) and may have been nocturnal instead of using sonar like dolphins. Many of its fossils have been found in Europe, with the first ever found in England.
Now, we know what you're thinking: How come this fish-lizard looks so much like a fish or dolphin? Surely this uncanny coincidence must be attributed to God's good will, or so the folks in Texas would say upon seeing this thing. After all, it looks like a lot like a mutant fish-dolphin cross.
The similarity in form is just convergent evolution at work. There is a certain torpedo shape that works very well in the water, especially for things that probably ate the same things as dolphins. Form fits function. Ichthyosaurus had the same function, and thus the same form, or vice-versa. We can't exactly go back in time and tell.
Cooler or creepier still is that the Ichthyosaurus gave birth to live babies. Little baby fish-dolphin things were found inside the fossilized belly of a larger fish-dolphin thing with no eggshells to be seen. This is a result of Icthyosaurus being a wholly marine animal. There's no need to move onto land when one can give birth to live young. It just makes things a bit creepier for people who think that mammals are the only livebearers.
The only truly notable appearance of an Ichthyosaurus in pop culture is a small fight in Jules Verne's novel, Journey to the Center of the Earth. As with all awesome monster fights, the size of the Ichthyosaurus was exaggerated. Also, we're pretty darn sure the earth's core is molten, now.