As a current DM making his own campaign I'm having trouble coming up with random encounters. I want my players to feel immersed but also feel fear. Suggestions would be really helpful!

Look up a monster in dnd beyond, pick your poison, and then choose which one you are interested to use against your party as a challenge. If the environment matches the monster then use that. Also it really depends, If your focus is on horror use horror themed monsters but you specifically stated fear, all monsters are scary if the party isn't in the right level.

Quote from Lolths_Bane >>

Look up a monster in dnd beyond, pick your poison, and then choose which one you are interested to use against your party as a challenge. If the environment matches the monster then use that. Also it really depends, If your focus is on horror use horror themed monsters but you specifically stated fear, all monsters are scary if the party isn't in the right level.

 I'm really trying to focus on the horror elements. Thanks for the help! 

Do you mean horror as in Gothic horror?  If so, pick up any Ravenloft book (or look up old Internet threads from fans of Ravenloft).  Do you mean insanity/unspeakably bizarre horror?  Read some H.P. Lovecraft, pick up Volo's Guide to Monsters, and go to town with Mind Flayer pets. 

Quote from Theren_Dotsk >>

As a current DM making his own campaign I'm having trouble coming up with random encounters. I want my players to feel immersed but also feel fear. Suggestions would be really helpful!

Nothing should be truly random. It doesn't mean that everything needs to be connected to the main plot, but everything should have a very specific purpose. Think of the themes and motifs that you want to build into your campaign and work from there. What kind of statements do you want your world to say? Now how can you say those things without literally saying them in words?

e.g. Is your post magical apocalypse world built on a theme of hopelessness and desperation? Well then plan an encounter of bandits who are using some sort of diabolical magical device to ambush the players. They're not evil, just starving and the artifact they are using takes a shocking toll, but they deem it worth it to feed their families.

My advice is take pre-existing stuff. 

• I ran a destroyed caravan surrounded by scarecrows in COS. Some were bodies strung up by scare crows and some were the scare crows monsters. A more advanced version might be using this mini adventure as like a point of interest https://www.dndbeyond.com/posts/334-halloween-encounters-the-haunted-cornfield 
• A vistani caravan is a good random encounter too. Get your fortune read and maybe trade. They work in allot of settings you can even steal the vistani camp map from COS
• Hags are always nice to have around to give the players and option for a dark deal to solve a problem. You can again steal one of the hags from COS, the mill or BaBa lysaga's hut as maps
• You can plop down a haunted house from salt marsh, van richtens guide or COS

These all make good little distractions which are a bit more than. You see some wolves. Otherwise throw in some story things. Refugees, traders, secret werewolves, dead bodies, graves and alike. 

If you truly want your characters to feel immersed, set up some non-combat ones.   Night time does not automatically mean monsters.  

Some ideas:

• Lost child
• Loud storm that wakes everyone
• Weak creatures charging through camp for their own purposes.  think a dog chasing a cat, insect swarm, bats,
• Mischevious fey playing tricks on the party

Then do call backs for it later.  If the child is killed by the party, have them meet concerned parents looking for her/him.  If they do not kill her/him, ret con the child into a vampire / ghost / whatever  to feed back in the horror.

That storm?  Was actually giants fighting.    The creatures were wizards in disguise, etc.   

Thanks so much!

a kobold bard walks into camp, before throwing knives at everyone and then disipearing

Hey hope you don't mind but me and my friend will bounce a few ideas for you off of each other...

To begin with I would definitely build up a atmosphere at night-time, and build up intensity during daytime. For example, at night, make random noises and some increasingly powerful creepy creatures. At day make it similar but just build up the number or power in general.

Maybe if they are in a corn field or something they could be travelling through and loop back on them selves and notice that a scarecrow or something is missing? Maybe even they see one moved a solid inch or two???

As for daytime, maybe just a couple of goblins to start with then get more goblins, then maybe some goblins with orcs, then more again, then ogres and so on???

With the scarecrows maybe you could have something like a high perception character notices something eerie about the corn field. Or maybe you could hint at the scarecrows being alive. And again with the daytime. Maybe don't make it quite so obvious that it is increasing. Like maybe make it happen at a village that they've been sent to protect. For example maybe a hunter notices tracks around the edges of the village and estimates about 50 goblins but then either its way less or way more??? Or maybe it's actually something else completely?

maybe the scarecrows are stalking them. they could see random straw in places, or stuff is missing or moved. and then someone dies randomly like an npc that was travelling with the party?

As for the daytime again. Maybe the party thinks they're just about to win a large battle with goblins when a large cavalry of ogres or other powerful enemy appears?

Or maybe something else happens like all the goblins are actually already dead and they rise again and the party finds out that a necromancer is behind everything???

Those are great ideas Luna! I'm kind of burned out for ideas now though, hope that was helpful!

Quote from Theren_Dotsk >>

As a current DM making his own campaign I'm having trouble coming up with random encounters. I want my players to feel immersed but also feel fear. Suggestions would be really helpful!

 One encounter I'm doing for my campaign (Most of the encounters are like this, it's the entire campaign) are different types of oozes deep underground. I even threw in a couple of purple worms underground, making some tremors for a little extra terror.

They do end up fighting the purple worms and some point.