Troll ducks

Nobody knows why evolution brought this species into being, as a silicon-based wildfowl is too heavy to either fly or swim. It is thought that they sink to the bottom and walk accross underground lakes.

However, the lake of pure mercury (re)-discovered by Rincewind in Interesting Times opens up a tantalising possibility. What if this is not an artificial construct, but a natural feature utilized by One Sun Mirror for his own ends?

If such liquid lakes exist naturally on the Disc, then an extremely dense medium which will neither poison a troll duck nor permit it to sink would be the natural breeding ground for the species...