A grelth is an Andorian arachnid.
In the year 2267, Sahirn P'Thall referred to Aaron Cole as a grelth. (ST novel: The Case of the Colonist's Corpse)
In 2376, after Thirishar ch'Thane encountered his mother Charivretha zh'Thane aboard the USS Defiant, the nervous Shar imagined he could feel a grelth spinning a web in his stomach. (DS9 - Gateways novel: Demons of Air and Darkness)