I will assume you are using 1.13 or 1.14. If that is not the case, let us know.
Adding entities is, unfortunately, not simple. There are many steps:
Create the Entity class - depending on what behavior you want, you will choose which Entity class to extends
Register the Entity by handling RegistryEvent.Register<EntityType<?>> and using EntityType.Builder
Create the Entity model (or extends an existing model class) and texture. This has not changed since 1.10 or 1.12 so you should be able to find tutorials about it
Register the Entity model. This has not changed since 1.12 (you use a IRenderFactory and register client-side only after registering your entity)
#1 - Choose an Entity Class to extend based on what you want it to do. If you want a basic zombie-like mob, extend MonsterEntity or even ZombieEntity. If you want a passive mob, extend AnimalEntity. (If there is no "baby" version, extend CreatureEntity). There are tons of options but you should choose whatever is most similar to your desired entity
#2 - Take a look at this post for the registration part of things.
#3 - Look up a tutorial for making entity models. Back in 1.7.10 I used Techne but I know there are much better modeling programs out there by now. Make a Render class to use the model. See this GitHub for examples.
#4 - Call RenderingRegistry.registerEntityRenderingHandler(MyEntity.class, MyEntityRender::new) after registering the entity itself.
I guarantee that you will have many, many problems along the way as Entities are one of the harder parts of Minecraft modding. I haven't even mentioned AI and Goals, Tameable mobs, Rideable mobs, etc.
Good luck.