Dr. Gray

Dr. Gray is the main villain of the Goosebumps book "Egg Monsters From Mars" - he was a scientist who was keeping "egg monsters" in a large hidden lair and was approached by Dana when he found one of the last "egg monsters" to arrive on Earth from their native Mars.

Dr. Gray intended on keeping Dana for experimentation since he had been in close contact with the "egg monsters" and left Dana in conditions where he almost froze to death, being saved by the "egg monsters" themselves, who kept him warm.

The "egg monsters" then worked together to overpower Dr. Gray and allowed Dana to escape but by the time he convinced his parents to come to Dr. Gray's lair both the scientist and the "egg monsters" had disappeared.