As we all know, your first line of defense when a giant reptilian monster emerges from the sea is an army of fighting robots a la Pacific Rim. If fighting robots are not available, sheep make a good substitute. At least that's the premise of Monster vs Sheep.
The idea is that the monster wants to smash the city, but you can stop it by fattening it up until it can't walk anymore. Just keep shoveling sheep into its mouth as it walks until it tips over. At the same time, you need to toss humans out of the way because apparently they're too stupid to run away from the monster.
The graphics are simple but rather fun. The monster stomps around and gets noticeably fatter as it fills up on sheep. It also throws up a lot. The game is $0.99 and on the matter of in-app purchases, the description has the following to say: "Just kidding. Pay once, and play forever!"