He who shall train the Horse to War Shall never pass the Polar Bar.
We're still dealing with animals here, but this time not with direct cruelty to animals… exactly. No one is, apparently, torturing this horse—it's just being trained to fight, like humans do. But, in Blake's eyes, this is a crime, because it's twisting a peaceful part of nature—horses—toward a destructive end.
One of Blake's biggest fans, the critic Foster Damon, claims that the "polar bar" line references The Odyssey, where humans are said to enter the underworld through a gate in the north. (Blake also talks about a gate in the north leading to the spiritual world in his poem The Book of Thel.)
So, using Damon's idea, Blake means that, if you're messing with nature and turning peaceful animals or things toward violence, you're not going to get to enter the spiritual world.
Also, this is another case where the rhyme is probably based on old-time pronunciation ("war" and "bar").
The Beggar's Dog & Widow's Cat, Feed them & thou wilt grow fat.
Now, Blake is switching things around a little. Instead of saying something like, "He who Punches a Pig in the Face / Never shall Finish the Stockcar Race" (not that that's a real one), he's putting a positive spin on things.
Feeding animals belonging to people who are in tough situations (beggars and widows) will help you make sure that you're well-fed. In the same way that cruelty comes back at you, generosity rebounds back too. You "reap what you sow," in other words.