The plot thickens, I had plenty of time to return to the Castlemilk History Facebook page again today and spotted there was another sculpture originally right next to Falstaff, and it was of a sheep!
The stone beast apparently represented Castlemilk's own distinctive brand of Ovis. You can see the Falstaff statue on the right of the postcard and Castlemilk House in the back ground. If there was something as eccentric as a sheep sculpture sitting there I do wonder if there ever was a turtle one placed by the lilly pond? Odd that there is no reference to such an amphibian character on the extensive Facebook history page. The lilly pond was drained in the late 1960's following a series of tragic children drowning accidents, one of the old buildings next to it had by then become an orphanage.
You're right Vinegar Tom about the apparent "cast" look of the Falstaff thing, no trace that a chisel ever touched it. Here's another pic of it, just keeking through on the left.
If you want to see what it looks like in real life apparently it still exists! It is now at the Childrens Zoo at Calderglen Country Park at Torrance House East Kilbride. This photo was shot last year.
A million thanks to all the original posters on the Castelmilk History Facebook page.