From the wiki:
https://stardewvalleywiki.com/Monsters
Here's another thread that you might want to check:
https://forums.stardewvalley.net/threads/wilderness-farm.9370/
Monsters don't spawn super frequently, often times you'll have to go looking for them. They take a little while to kill when you only have a Scythe (the best weapon before Day 5), but their drops can be handy (such as Mixed Seeds from Wilderness Golems).
If you want to, you can check out the links in the table to find out what they each drop.
If you're going to use the Wilderness Farm, I suggest moving the Greenhouse somewhere a bit less out of the way (like
@jele77 did). Having the Greenhouse take up the only big square of land drove me crazy, but 1.5 fixed that well, and in my opinion it's now a much more balanced prospect compared to the other farms!
Hope this helps!