why hänellä and not hän~?
if it read "hän on poni" it would mean that he is a pony. we use the -lla / -llä -ending when somebody has something to make the distinction between to be and to have, because both of those translate to "olla".
Because adessive case is necessary for expressing possession.
Hänellä on yksi poni? why not?
A pony = poni / One pony = yksi poni
Essentially the same thing, but as a translation not.
I'm a little confused about why "on" is used here? I thought it was supposed to mean "is" like "Tämä on poika - This is a boy". But in this sentence it's used as "has"
Finnish doesn't have a verb for possession like to have in English. Instead we use the adessive case (-lla/-llä) combined with the verb olla. Thus
he has - hänellä on
I have - minulla on
They have - heillä on