These character descriptions are from the film, rather than the book.  Also, as an interesting tidbit, according to the Webster's dictionary, "tod" is a Scottish term for 'fox.'
Main Characters, The Plague Dogs(film)

Having always been a laboratory dog, he is cynical and critical of humans, suspecting them all to be "whitecoats." Before escaping from the lab with his friend, Snitter, he was subjected to a repeated test of endurance, in which he was immersed in a large tank of water with no way of getting out, swimming until he passed out from exhaustion, after which time the scientists pulled him out. As such, he has an intense fear of water.

Once owned by a kind man, he was brought to the laboratory after his owner was killed by a passing vehicle as he tried to get Snitter off the road. He insists to Rowf that the whitecoats 'are not masters.' Whilst in the lab, he underwent an experimental operation to unite the conscious and subconscious parts of his brain, giving rise to frequent, dreamlike delusions which seem to be the reliving of past experiences, particularly ones related to his former master.

The Tod
This cunning, clever trickster meets Rowf and Snitter in an abandoned mine after their escape, and while Rowf is wary of him and has trouble understanding his Geordie dialect, Snitter sees him as an invaluable companion in their quest to survive. In exchange for part of the kills Rowf and Snitter make, the Tod does his best to instruct them on how to be 'wild animals.'