Cassandra
Eeyore
calamity howler
crepehanger
cynic
damp
dampener
damper
defeatist
gloomy Gus
grouch
grump
malcontent
malist
miserabilist
negativist
nihilist
pessimist
prophet of doom
retreatist
seek-sorrow
sourpuss
spoilsport
wet blanket
worrywart