The words canine, K9 sound the same but have different meanings and spellings. Why do canine, K9 sound the same even though they are completely different words?
The answer is simple: canine, K9 are homophones of the English language.
Of, relating to, or characteristic of the canids.
Of, relating to, or being one of the pointed conical teeth located between the incisors and the first bicuspids.
An animal of the family Canidae, especially a dog.
One of the pointed, conical teeth located between the incisors and the first bicuspids. Also called cuspid.
A trained police dog.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.