beware sea donkeys

#1
had a precious encounter with one yesterday. dont wade over your waist if a bull is present chasing fish. he might just let his presence be known to ya. Ben
 

mr trout

Trevor Hutton
#2
I am truly sorry for my naivety, but being from the east side, I have never run across a sea donkey. Could you humor we ignorant folk and explain what this is? Thanks - Trevor :CONFUSED