It is a nickname for Barnard 138 dark nebula in the star rich area in constellation Aquila (Eagle). Actually this is not very well defined dark nebula, that hardly resembles this animal. If you increase contrast to almost maximum, then its shape is more obvious, however when someone wants to keep relative brighthness all over the frame, then Black Lizard is not really black. Only few shades darker than surrounding star field. It is supposed to be about 2000 light years away, and can be spotted with medium sized binoculars under really dark sky. Any light pollution or any higher magnification will make this dark dusty invisible.
There are few more dark nebulae in the frame – two more from Barnard catalogue and several from LDN catalogue – you may follow them at the map below (map comes from http://www.forumastronomiczne.pl/index.php?/topic/8715-obiekt-tygodnia-7102015-barnard-138/ ). And also a few shining deep sky objects were captured – two globular clusters and PK 36-1.1 aka Sh2-71 planetary nebula – tiny reddish egg shaped blob in the upper right frame part.
Image was recorded in Anticiana, Toscana using Samyang 135/2 ED lens and QHY163M camera on Smart EQ Pro mount. It is 2.5 hours of total exposure with 2 minutes subframes through LRGB filters.