It is in campaign, as you can see the Master Chief get into it.
Source:
1:40 - 1:41
http://www.youtube.com/watch?v=ttcHY3CsSbo
It is in multiplayer matchmaking as a vehicle that constantly respawns and can be destroyed, so that means it's placeable, so it's in Forge, and because you can edit the respawns (placing them, timers) of vehicles, therefore it is in custom games too because they are an edited Forge experience. It's not like the Elephant or Mammoth, those are too large and never respawn. No reason why it wouldn't be an expensive placeable vehicle, just like the Scorpion.
Source:
3:39 - 3:49
http://www.youtube.com/watch?v=UEIB2txWyeo