I'm using Minecraft Forge 1.8-8.0.12.1018-1.8, and I want to make a custom mob. I have my Techne model and textures. How would I go about creating it? The tutorials on the Wiki are totally outdated, they aren't even for 1.7.

Well, I don't know about 1.8 but I know how to do it  in 1.7.

How would you do it in 1.7 then? Do you have to deal with this RenderManager class?

No, I don't think it involves RenderManager. But you say you have a Techne model, that's a start. The second step is to get your entity class coded. What features do you want your entity to have?

I want it to be a passive mob, like a sheep or cow, but cannot be tamed. I think I have the basic mob classes right, but I have no idea how to render it.

I want it to be a passive mob, like a sheep or cow, but cannot be tamed

 

Sheep, cows, pigs, and chickens can't be tamed.  They just are.  "Tame" is an aspect of hostile mobs, such as the wolf, ocelot, and iron golem.

You're going to need a Rendering class and register it in your client proxy.

I have a tutorial for 1.7 here: http://jabelarminecraft.blogspot.com/p/creating-custom-entities.html

 

Regarding making something tameable, you can do that by implementing the IEntityOwnable interface.  Also you may need to add some of the AI related to how it reacts when owner is attacked or attacking.

From what I've read in 1.6 and I presume 1.7, you'll need the actual Entity class, as well as a Render class and a Model class.  I seem to recall a Techne model can be imported to satisfy the Model part of the equation, and coding the mob's behavior will need to be done in the Entity class.  I believe you'll need to extend from EntityAnimal if you want a non-tameable class; EntityTameable would give you the behavior similar to Wolves or Ocelots.

 

You can look in net.minecraft.client.renderer.entity for classes that handle the rendering of some of the passive mobs.  I'm still pretty new at this, but from the reading I've done and videos I've watched, that's the basics of it.  I'm sure there's more specific information you'll need to get it right, but hopefully that points you in the right direction.