No description or picture is given of the Lesser-Spotted Squirrelserpent. Instead the Book details the Shortwing Squirrelserpent. We might assume that these two Squirrelserpents are similar in appearance, except perhaps the Lesser-Spotted Squirrelserpents is a tad smaller ("lesser") and spotted.
Because the Shortwing Squirrelserpent is classified as a Tree Dragon, and because of the 'Squirrel' portion of its name, we might assume the Lesser-Spotted Squirrelserpent is also a Tree Dragon.
|“||"The Lesser-spotted Squirrelserpent" Hiccup said to himself. . . . Hiccup was so busy with this thought and with watching the Lesser-Spotted Squirrelserpent wheeling through the air in a lazy arc, that he lost his concentration, and his foot slipped on the crumbly edge of the window.||”|
|— A Hero's Guide to Deadly Dragons|
In turn, the Stealth Dragon who was supposed to get Hiccup from the window was likewise distracted by the same Squirrelserpent.