As a whole, I think you're doing a really good job. There are a few issues that come up here and there, but overall what I'm mainly seeing is that you're developing a good grasp of how these animals are each made up of relatively simple components, and how you can combine those components with clear, believable 3D relationships to create solid results tnat feel tangible.

There are a few issues I want to call out, so these should help keep you on the right track as you continue to move forwards. I noticed that a lot of these issues were present on one of your horse drawings, so I decided to point them out with some redline notes. You'll find them here. Let's go through the issues I pointed out in turn.

Firstly, when drawing your additional masses, you are largely doing a good job, in a lot of these you're thinking quite a bit about how they wrap around the existing structure, although there is a tendency to be a little arbitrary about where you place the sharp corners. Make sure that when you draw these corners, they are always in direct response to some other structure or form that the mass is pressing up against. Generally the form in question should also be defined (like the big shoulder bulk on the horse is), and the mass should conform to it tightly. In your case, the one I've pointed out doesn't maintain a tight relationship - instead the corner is a bit further out, making it seem kind of random.

Think about it as shown in this diagram. The additional mass is hugging and gripping that structure, like a ball of clay being pushed into a block of wood. Even if that block of wood wasn't explicitly drawn, you'd still be able to infer its specific nature based on the void forming in the ball's silhouette. If however you drew the ball without clear awareness of the block, it'd end up being sloppy and vague, and we wouldn't get a clear impression of what was pressing against the ball. For that reason, it's best to ensure that all the forms at play are defined on the body.

In cases where you don't have a clear form to press up against, and therefore no sharp corner, you can use a smoother transition as shown on the back legs.

Of course, when you draw these additional masses, as with any form, make sure you draw it in its entirety. You appear to have tried to introduce a mass along the horse's chest, but you cut it off where it passed behind the front leg.

Moving onto the hooves, you drew these to be quite flat. It's understandable, as this drawing is seen from the side, but you need to always remind yourself of the truth of things - the objects we're drawing are three dimensional, and you cannot physically just see something dead on from the side, orthographically. So remember to establish these forms as being three dimensional, even if you're only drawing their silhouette. You can still use corners here to imply how the form exists in three dimensions.

The last issue I wanted to call out is probably the most prevalent across your drawings. It is largely that your head constructions aren't quite as well structured as they could be. This is not exactly your doing - the current head construction demo that is made part of the core lesson material (the tiger head) is quite old at this point, and my strategies for teaching this stuff has improved. While I haven't yet been able to incorporate it into the core lesson material (I'm currently working through back from Lesson 1, updating all of the content as I go), I do have a more comprehensive explanation on how to approach head construction here in the informal demos page. Make sure that you refer to all sections of the lesson, otherwise you might miss some valuable material.

Give that explanation a read - it goes over how to think about the head as a bunch of separate pieces all wedged solidly together, and how the eye sockets can be used as the first step to separating the otherwise smooth surface of the cranial ball into a series of complex planes.

Anyway, all in all I am very pleased with your results, so I'm going to go ahead and mark this lesson as complete.