This one is quite sweet in a faintly Pauline Baynes kind of way but he seems to have a bit of a problem finding his assets.
There’s a faintly Victorian one here, but it’s by David Delamare, 2001.
I guess there’s no Victorian ones because neither Simeon Solomon nor the women Pre-Raphaelites and Symbolists turned their hands to the subject.
There are some depictions of mermen in Wikimedia Commons. There's also a weird cryptozoological specimen (warning: disturbing photo of dead merman) but it looks as if it's made of papier-mâché to me, or perhaps photoshopped.