My hook for the Bulette was a scenario in which the PCs must spring a trap for a Bulette that's been terrorizing a rural community. There isn't much else you can do when it comes to planning an adventure around a monster that doesn't have a lair, and was essentially designed to be most effective as a random encounter. So we can't do much else with an Ankheg, except add it to the original scenario. Here's the hook:
- The PCs are employed by a monstrous extermination service, which uses creative methods to trap rural pests. They travel from one community to the next, taking assignments through a dispatch.
Crap, now I wish I had the time to try this out in one of my own games.