Bsrhrki

The dog lifted its hind leg towards the fire hydrant.

I am not sure I like using the preposition "towards" in this sentence. I prefer "on", because it sounds nice, but I am not sure if it's correct at all. Is it correct? Why?

"On" doesn't really work here, but "onto" does. Still, they really don't have the same meaning.

The dog lifted its hind leg towards the fire hydrant.

The leg is now nearer to the fire hydrant than before, or nearer than the other parts of the dog. This is what dogs usually do when they want to mark their territory by leaving their smells behind.

The dog lifted its hind leg onto fire hydrant.

The leg is now resting on the fire hydrant. The dog is possibly standing with three legs on the pavement and one leg somehow on top of the fire hydrant.

I think an aspect of this that @Wilson is missing is that a dog "lifting its leg" is usually used as a euphemism for a male dog urinating [on something].

In my experience in this 'non literal' context, it takes the same preposition logically as 'urinate' would take.

The dog urinated on the fire hydrant.

The dog lifted his leg on the fire hydrant.

When I read the example "The dog lifted its hind leg onto the fire hydrant" it seemed very strange to me. It sounded like the dog literally lifted his leg and placed it on top of the hydrant.

"Lifted his leg towards" sounds like the dog is saluting the fire hydrant.

So, to me, as someone whose male dog spends a lot of time lifting his leg on things in my company, "to lift his leg on" is the most natural choice.

At is for places:

The dog lifted his hind leg at the fire hydrant or when he got to the fire hydrant.

On and in do not work here.

Dogs generally just aim at the hydrant. They aim at the fire hydrant and hit the side of it, not the top. So on is out. Unless you have a really big dog.

So lifted at can be understood as aiming and peeing. Or it can mean that is the place where he lifted his hind leg.

Like: We stopped at the house for a cup of tea. :)

Looking for examples, I came across

The dog lifted its leg against the lamp post.

At any rate, it is an example from a reliable source.

And here's the example of using the preposition on in the context of a male dog preparing to pee.

As a side note, "hind" seems to be redundant in such a situation)