Hi again, Maggie! I see you're reading _Under the Dome_. :)
Question 1: adding "the" to this sentence would not change its meaning, but would not improve it either. It would actually make it heavier, in my view. "Horace was first" sounds leaner.
Question 2/1: I think "first" is an adverb, here, but I'm not 100% sure.
Question 2/2: "to" is part of the phrase "first to", which is usually followed by a verb in the infinitive. "I was first to answer her offer."
Question 3: "low to the ground" is a set phrase you can use to describe some sports cars, short-legged dogs (corgis, dachshunds, basset hounds, etc.), beds, couches, bicycles, etc. "To" in this phrase means the same thing as in "close to the ground".
Have fun reading!