If you do a little searching on the internet you might find the assertion that the Greek word Kerberos is cognate with Sanskrit karbarah, sabalah “spotted, speckled;” and, therefore, that it is related to our pet name “Spot”. This is a nice story, but like many nice stories, it is probably not true.
Pierre Chantraine lists this as a “doubted for good reasons”. (Here’s a link for a free download of the dictionary). I don’t really have a better suggestion, but I can quote some equally valid nonsense from antiquity.
Kerberos: From “karbaros” which is from having a heavy head. For the dog in Hades had three heads, as the story goes about the dog Kerberos.
Κέρβερος, παρὰ τὸ κάρβαρος, ἢ παρὰ τὸ τὴν κάραν βαρεῖν· τρικέφαλος γὰρ ἦν κύων ἐν ᾅδου, ὡς μυθεύεται κύωνος κέρβερος.
Cf. κάραβος (karabos) “horned beetle”
Also consider from Hesychius the Lexicographer:
Kerberioi: Weak-men. They also call the Kimmerians Kerberians. And some call their city Kerberia, but others call it Kimmeria. Others say that Kimmê is as place in Hades.
κερβέριοι· ἀσθενεῖς. φασὶ δὲ καὶ τοὺς Κιμμερίους Κερβερίους· καὶ τὴν πόλιν οἱ μὲν Κερβερίαν καλοῦσιν, οἱ δὲ Κιμμερίην· ἄλλοι δὲ †Κιμμη. ἔστι δὲ τόπος ἐν ᾅδου (λ 14).
I eagerly await other (and better) suggestions!