Go to the move relearner in Pastoria City, in exchange for a Heart Scale he should be able to teach Tyranitar fire fang.
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No, unless it's possible that it's hidden power is grass. If you wanted it to learn a grass type move to cover up its weakness, here are some alternatives: It can learn Ice fang and Fire fang which can be used for grass types or Thunder fang for water types. Source: pokemon.marriland.com
It can through breeding...
Drapion can learn thunder fang, fire fang, ice fang and poison fang Drapion learns thunder, fire and ice fang at the start, you just need some heart scales It learns poison fang at level 39
level 30
Level 65