Unicorn hasn't been available in awhile now. It was only available for max ranks in GW1 for a short window near the launch of GW2. Sometimes you can play and rank them up in certain portal events (like my friend did who wasn't max rank in GW1), but it won't be available outside of those events.
Both I think! My friend did that before the twilight chomper was added just to get max rank! I know twilight chomper didn't add to the max rank but I think you can use it to get to max rank in lieu of the unicorn chomper.
@Furbyvaca123 Correct. When they added Twilight Chomper, they didn't increase the level cap. So you can either get the twilight chomper or use the unicorn chomper in mystery portal events like my friend did, to hit the cap.
