loaches r cool: Your pic looks like a Synodontis eupterus or featherfin cat. I have one in my tank and it hides in a very similar spot. Behind a log.
They are relatively common in the trade and are quite variable. So bboru, yours might be the same. They have the characteristic 'sail' like fin and it usually looks a bit frayed on the edges with flag like extensions on the end of the rays. Hence the feather like description. It may also, unfortunately, be a hyvrid species. Lots of hybrids are currently being pumped out of eastern europe and are being sold as 'true' synodontis. But as everyone says, a pic would be great.
It could also be S. nigrita, which as the name suggests, is black. Not sure if nigrita have the sail fin though. Hope this helps
Happy Fishkeeping