"Polle is a horse, which can run really quickly."
Translation:Polle on hevonen, joka osaa juosta tosi nopeasti.
"Jolla" would be used to show possession. For example, in a sentence like "Musti on koira, JOLLA on kolme pentua" ("Musti is a dog that HAS three puppies"). Think of "jolla" like you would the other possessive pronouns ("minulla", "sinulla", "hänellä", etc) that all use the adessive (-lla/-llä) case. I hope this makes sense and helps.