A dog starv'd at his Master's Gate Predicts the ruin of the State.
Now, we go from the bird-world to canines, although we're still vibing on the animal theme. Also, we're not talking about freedom being crushed anymore, specifically. The speaker's focused on attacking good, old-fashioned cruelty. People starving dogs—that's pretty low.
In the same way that the caged robin is a symbol for freedom being crushed anywhere and everywhere in the world, the starving dog is a symbol of cruelty and injustice. Even though this is just one dog, its owners' neglectful and probably evil attitude is a tiny piece of the neglectful and uncaring nature found throughout society. They will also doom that society—which is why they predict "the ruin of the State."
(There's probably some personal politics going on here too, with Blake attacking the British political state of his own time.)
A Horse misus'd upon the Road Calls to Heaven for Human blood.
Like the dog from the last two lines, the horse is another example of victimhood and the consequences of pointless cruelty the world over.
Also, in the same way that the robin's cage puts heaven in a rage, the horse's ill-treatment calls for vengeance from the higher powers of the heavenly world against human beings. The abused horse is a living example of the twisted nature of human beings—the way they've become warped into cruelty when they should be feeling pity and mercy.
Form note here: You may think that rhyme scheme that's been holding up till now (check out "Form and Meter") is broken with these lines. Back in the poetic day, though, words were pronounced differently ("again," for example, was pronounced with a long "A" vowel sound). So, it's more than likely that "road" and "blood" might have been a perfect rhyme in Blake's own time.