Have creatures ever evolved to become less intelligent?
Unfortunately, measuring intelligence (assuming we would have a strict definition) is not a thing that can be done from fossil record or genetic data. The only thing one can do is to compare brain mass (or at least size of the cranial cavity) from fossil record. In this regard, no need to go very far for an example, humans are an example. From wikipedia


  The evolutionary history of the human brain shows primarily a gradually bigger brain relative to body size during the evolutionary path from early primates to hominids and finally to Homo sapiens. Human brain size has been trending upwards since 2 million years ago, with a 3 factor increase. Early australopithecine brains were little larger than chimpanzee brains. The increase has been seen as larger human brain volume as we progressed along the human timeline of evolution (see Homininae), starting from about 600 cm3 in Homo habilis up to 1500 cm3 in Homo sapiens neanderthalensis which is the hominid with the biggest brain size. The increase in brain size topped with neanderthals; since then the average brain size has been shrinking over the past 28,000 years. The male brain has decreased from 1,500 cm3 to 1,350 cm3 while the female brain has shrunk by the same relative proportion.


Note however, that wikipedia does not cite any reference and I failed to find one!

Note, also that one should not confound cranial cavity (the thing that is actually measured) with brain size and more importantly no confound brain size with intelligence. Wikipedia goes on saying


  However it is argued that another essential element of brain evolution in humans is rearrangement (Hoffman et al. 2004). Larger brains require more wiring, but more wiring can become inefficient (Hofman 2001). The brain has therefore become reorganized for efficiency. Furthermore, the average body size of nethanderthals was larger which led to bigger brain size (see Brain-to-body mass ratio).


I could not find the Hofman papers but I probably just did not look hard enough (and am a bit tired right now)! One might want to read Healy and Rowe 2007 for a criticism of the inference that can be done from measures of cranial cavity.