In correlative table (tabelvortoj), "u" denotes individual (one), and "o" denotes thing. Tiu(n) is "that one [i.e. horse]" and tio(n) is "that thing" or simply "that".
I printed out a correlatives table to keep by my computer for reference (I also studied the quick course at Memrise "Ace Correlatives Like A Native"). There's a good table inside this excellent, short grammar list at http://esperanto.bretonio.free.fr/dokumentoj/pakeo-en2.pdf and Wikepedia also has a correlative table at https://en.wikipedia.org/wiki/Esperanto
Hope this helps you.