illithid
This is an illithid, also known as a mind flayer. Their physical attacks are not very strong, but as they attack you, they latch their tentacles onto your head, and when all four are attached, they extract your brain, killing you instantly. In the meantime, their mental powers keep you constantly slowed, allowing them to catch up to you with no problem.
And if you think that bringing your little dogs and cats to the party will help you, you would be rather wrong. As illithids are only interested in eating human brains, they have another use for such simple-minded creatures: turning them on their masters. Illithids will take a few moments before dealing with you to dominate the minds of your animal companions, turning the tide against you. Of course, their control over them is based upon their mind, so if you kill the illithid, your not-so-faithful friends will return to your side.
Important: Due to the way tamed animals save their data, any animals that are dominated will stay that way if you close the game - or it crashes - while around an illithid. This is being worked on, but I can't say how long it will take. Engage illithids with pets along at your own risk!
Illithid are from Dungeons & Dragons. In early monster manuals the name mind flayer (mindflayer) was used for the monster entry, and it was noted they were also called illithids. Later the illithid became the primary name with mind flayer list as the alternate.