Groundhogs are a random Varmint that spawns when crops are harvested. They are therefore the easiest, though they do require capital for spawning (i.e. you must have Coins first in order to purchase Crops). It is rumored that Groundhogs like Potatoes the best.
If you have the goals Teach Hunting and Scare Bears simultaneously, you can use your spouse or child to scare the bear, and it will count for both goals.
Using your child or spouse to fire a varmint cannon does not count as clobbering or scaring the varmints required to accomplish this task.