Tobonga

First appearance: From Hell It Came (1957)

Nuclear radiation turns a prince who was executed when he was wrongly accused of murder and buried in a hollow tree stump into a revenge seeking walking tree known as a Tobonga. The Tobonga goes on a rampage killing everyone who gets in his way including the guy who actually committed the murder. Can anyone get close enough to drive the knife protruding from the monsters chest all the way through the monster's heart thus killing it?

I don't think there's enough monsters nowadays that are the result of radiation and revenge.

Paul Webster

First appearance: Alligator People (1959)

Under hypnosis a lady remembers a weird episode from her past where she reveals that she was once married to a man who was turned into an alligator because of science. 

Glenn Manning

First appearance: The Amazing Colossal Man (1957)
This appearance: War of the Colossal Beast (1958)

In the Amazing Colossal Man, Lt. Col. Glenn Manning was hit with radiation. Instead of contracting cancer and dying he grew to fifty feet tall, went insane and goes on a rampage. The military eventually knocked him off the Boulder Dam, killing him. Or so they believe. Upon receiving reports of strange food truck robberies in Mexico, his sister (who according to the first movie didn't exist), an army guy and a scientist decide that Glenn is alive and committing these crimes. Turns out they are right Glenn didn't die in fact plutonium radiation made him ten feet taller, melted off half his face, stupider and addicted to loaves of bread. Our group of intrepid heroes capture Glenn, drug him and bring him back to the States in order to cure him, or something.
Back home Glenn wakes up and goes on a rampage in Los Angeles. What can stop and Amazing Colossal Man?
Coincidentally (or not) the man who made this movie Burt I. Gordon's initials spell BIG.