When Holmes need to test the pills that were found in Joseph Stangerson's hotel room he asked Watson to fetch the landlady's terrier that she had asked to be 'put out of its pain'. (A Study in Scarlet)

 

"A queer mongrel with a most amazing power of scent. I would rather have Toby's help than that of the whole detective force of London" is how Holmes described Toby to Watson. Toby belong to Sherman the bird stuffer. Toby was "an ugly, long haired, lop-eared creature, half spaniel and half lurcher, brown and white in colour, with a very clumsy, waddling gait." (The Sign of Four)