[i]Although it is never seen in the game, support conversation between Cherche and Nowi reveals that the wyvern mounts used in Fire Emblem: Awakening are able to breathe fire. [/i]

From de wikia.

Wyverns in fire emblem aren't Wyverns from dragon age, they're devolved dragons. Get [i]your[/i] facts straight. :p

No where in fire emblem is there mention of wyverns using venom. At least not on the wikia.