I think Icu4dzs is giving some good common sense advice, undoubtably also from REAL experience.
I am going to take a big chance, go out on a limb, and run my mouth off some more about the question here.
First, a couple of caveats: I am no gun guru, I don't spend my days at the range discussing ballistics and don't have a room full of guns. I have just had them around me all my life as a tool for doing certain jobs.
I don't really know your situation, and your exact situation will determine what decsions you should make concerning guns. I will just have to make a coupe of safe assumptions.
I want to start this with the snake question. A snake will only come into your yard for a good reason. There is either a good reason right there in your yard, or it is just passing through on its way to something it needs. Start here by removing all needs for it to be here if you can. Try to exclude it from your henhouse, eliminate brush piles, vermin, so forth.
Next, learn your snakes. They are your neighbors, so like it or not you will have to deal with them. Get a first rate guide like Peterson's Reptiles and Amphibians. Where knowledge begins, fear loses power. Besides that, you will soon learn how to quickly identify what snakes are no threat at all. I also believe a couple of Colubrids in your chicken area, particularly of genus Lampropeltis (kingsnakes) will not only keep your rats down but will also discourage the scary guys from moving in.
Now let's move on to the good stuff - GUNS
I have said that I only will kill an animal if I have to. I also will only cut down a tree if I really have to - and I own a few axes and a chainsaw in case that happens. I also own guns in case there is a need for them. I feel that a good gun or few are just as necessary tools on a homestead as a shovel or a pocket knife. Again, not a whole military level arsenal, just some well thought out choices.
Let me assume you have a semi-rural place in the lower 48, and not a whole lot of experience with guns. I also guess you aren't going to take up deer hunting this year. You also are very unlikely to want to sink a whole boatload of money into guns. If these guesses are way off, sorry no offense intended.
Okay, you can get by with one or two guns. Unless hordes of zombies come up your drive, this should do it.
I would look for a good solid, basic 22 rifle. Marlin makes a very good auto at a very good price - the model 795. I have one and it is a good rifle. You can pick them up at walmart for around $140 here, but your prices may vary. Marlin also has a $25 rebate on them right now. There are better rifles out there, and cheaper ones, but no better rifle at this price, especially not one that actually works like this Marlin.
Next you may want a gun just in case the most unwelcome event takes place. I am talking about a home defense gun. I would recomend a 357 revolver, from some maker such as Charter Arms, Taurus, or Ruger. You don't need to drop the mega$$ that Colt or S&W want. A revolver can sit loaded for ages with no damage to a magazine spring. You can also safely store it with the hammer down on an empty chamber and still have 4-6 rounds (depending on the model) available at the squeeze on the trigger. As to that caliber, 357 will kill any intruder you may have in the lower 48 at close range, so long as you hit it and hit it well. Recoil is a little harsh, but most people can handle it. You're not being asked to enter shooting competition with it, just get proficient.
You can also load up your 357 with 38 special ammo for practice. It is cheaper and less recoil. I think those revolvers are running in the $400 range right now.
If you only want one gun that can do both of the above, there are probably 2 options.
A lot of people will recomend a good shotgun, such as a 12 gauge pump. The Remington 870 is the flagship standard, although other people like Mossbergs and some of us like Winchesters. I have had all of them. This is a very good option, as you can load it up with buckshot or slugs and kill anything up to a bear (maybe a grizzly with very good shot placement) or birdshot, squirrel shot, so forth.
But be warned of a couple of things. One is that ammo is a bit more expensive. I think you will be paying nearly $1 / round. That adds up in practice. The other is RECOIL. A 12 gauge kicks hard. Even I, after shooting them all my life, tend to flinch up after a few rounds.
The other option is a pistol caliber carbine. The Marlin brand here is very good, but the 44 magnum caliber may be more than you want and the 357 magnums are really hard to find, especially at resonable price. Look at the Rossi brand. I think they fall short of the Marlin in quality, but the price is much better and they are still of at least acceptable quality, probably plenty good for your purposes. You can load them up with 38 as well, and pop varmints if you really must.
Just a couple more humble suggestions....
Stay away from the military guns. This is a public relations thing. The public has been taught that "assault weapons" are only possessed by crazy militia types or gang bangers. You can figure the rest of that out for yourself.
Whatever you get - practice. Practice and then practice some more. Not only will it make you a better and safer gun owner, but it can be fun. A day at the range with a 22 rifle and a big box of ammo is about the most fun you can have with your pants on.
As to guns for hordes of zombies, well that is an entirely different conversation altogether.
I'm not touching that one.
