Pets. They are wonderful creatures that come in a variety of different shapes and sizes. You've got cats, dogs, fish, and more and I believe all of them can help your mental health one way or another.
Starting off with fish, having something as simple as a beta fish can help your mental health out a great deal. Life is stressful and constant, never letting you take a break and having something to care for and talk to (even if the fish can't understand what you're saying) can help you out immensely if you feel are feeling alone.
Dogs are great because it is a pet you can actually hold. Dogs have a variety of heightening senses and one I've noticed with mine is his ability to know when I'm feeling down and he will come up to me and just cuddle up with me and check to see how I'm doing and just sit there with a smile on his face.
Dogs are also great to play with when you get done with a busy day of college or a terrible day at work or when you're just feeling out of it. For those of you who want a more laid back kind of pet that won't be as up in your face, cats are great.
Cats are interested in that one second they don't seem to care about you whatsoever and the next second they just cuddle up right next to you and purr and just want to give you all the love they can muster.
An interesting pet that I used to have when I was younger was a pet red ear slider turtle named Crush (yes as in the Finding Nemo Turtle). Crush was a wonderful companion to have around. I used to have a lot of anxiety and a lot of mental demons when I was younger and Crush would help me through those tough times.
I highly suggest having a turtle if you struggle immensely with your mental health. Watching in their tank swim around and cleaning their shell is some of the most therapeutic things I've ever experienced.