I asked a taxidermist friend with extensive experience of animal anatomy to take a look at the picture of the swamp thing, his reply is below:
The way I see it there are basically two possibilities here
A. Its certainly not a bird the skeleton is far too "heavy set" for that. My hunch is that it is the skeleton of a medium sized dog something like a bull terrier. its not a cat they too are far finer boned. ...........
or ............................
B. Failing that it could be a strange and, as yet, undiscovered species of alien life which has heroically solved the problem of space time travel, made its way across the galaxies travelling through the millenia, survived re entry only to land in a pond in soggy england and discover that it never learned to swim !!
We may never know the truth. Though for my money B. has the edge
have a good day