Any animal in the subkingdom "Parazoa" that includes sponges and the trichoplax. These animals have no nerve cells or brain whatsoever. If you are talking about animals that do have a nervous system, it would probably be the Nematode "Caenorhabditis elegans", or "C. elegans" for short. These creatures only have 300 neurons in their entire body.
However, your question could also be interpreted as "What animal makes the worst choices". Because, while C. elegans has 300 neurons, that's just enough for it to survive and thrive in it's environment. If we are going by that logic, I'd say us, Humans. While technically we are the most intelligent animal on Earth, we as a whole have made terrible decisions that have both destroyed our environment, and ourselves. We are one of the few animals that kill our own species en-masse. Even so, this behavior is not unique among Humans. Certain ants will kill other ants of the same species in wars between colonies. I now notice that I'm going off on a bit of a tangent, so to wrap it up, the answer really depends on what you mean by "dumbest".
the stupidest mammal will have to be the Hammer head shark, it looks funny and it swims funny
... get your facts right, people. sharks are NOT mammals.
The bush turkey. Has to be seen to be believed
Man
Probably the shrew or sponge
The Arctic
Love
spot spotnick
aidil jkr
It depends to the owner and how they think of their dog
Rantanplan
sbdacashcsh means the most stupidest person in the world
Ur the most stupidest person in the world!!!!!!!!!!!!!!!!!!!!
There are many people in the world who believe that they are the stupidest person ever. This is not true.
Boys have pelvises because they are the stupidest in the world! By, Someone You Don't Know
The world's "greatest" drinker would, by definition, also be the world's stupidest drinker. There are many who vie for the title, but no one has yet been crowned.
$2.50 i hate India the stupidest contry in the world