It's not getting enough sunlight. As you can see, the central leaves are pale and limp. They will green up in brighter light. These leaves won't firm back up, but they'll eventually get covered up by new growth. It's not that bad at the moment. Just give it some time and you won't even really notice it anymore.

When mine are actively growing, I give them direct sunlight. But when I bring them back indoors for the winter, they hardly get any sunlight at all. I don't even keep them near any of my windows. They just sit on my shelves and practically get ignored all winter long. I rarely water them during their time indoors; perhaps once a month. Since their soil gets bone dry, I place them in a slightly larger bucket and let them soak for about an hour, so that it's soil gets thoroughly hydrated again. Then I drain the excess water away, before forgetting about them for another month. You won't want to water them very often, because they'll try growing newer leaves, which will cause them to grow like your's are, pale and limp. They are great at storing water in their trunks, so they'll do just fine being kept more on the dry side during fall & winter. When spring rolls around, mine start growing new leaves again. That's when I move them back outdoors, so they can get all of the sun that they could possibly want.

As for the browning tips, are you thoroughly watering its soil when you water it? You have to flush the soil with plenty of water, until it's draining from the bottom, or else minerals can eventually build up in the soil and cause all sorts of problems, such as brown tips. Don't prune these tips, because you'll only cause the pruned tips to brown even further. Best to just leaves them alone, to minimize any damage.

Yea I figured it wasn't getting enough light, but since I just got it I figured it wouldn't need a lot of light for the winter. I've only watered it maybe 3-4 times so far and each time I thoroughly soak it. The browning tips are mainly what concerns me at this point because there are quite a few and it's actually more than just the tips, it extends several inches down the leaf.

When I repotted this much smaller one recently, it had an extensive root system. I had to trim a lot of roots to put it back into the same pot. (That was soon before the Ficus cutting was added.)

Is it possible the lack of light caused the leaves to turn brown?

I doubt it. If any leaf tips did turn brown from the lack of sunlight, it would be the lower leaves turning completely brown as the plant shed any older leaves in order to try and conserve energy. If only the leaf tips turn brown, then it's most likely from poor soil or poor watering conditions. Ponytails can easily tolerate low light levels, but not while it's actively growing new leaves. Those tender new leaves need to be exposed to bright, preferably direct, sunlight while they're growing, in order to hold their shape. Once they've stopped growing and have had the chance to harden off, then they don't mind lower light levels. Mine are still outside for the time being, but they've finished growing all that they're going to grow this year. I plan on leaving them outside for as long as possible, before I bring them back indoors and cut back on their water until spring. During their time indoors, they hardly receive any sunlight whatsoever, and they do just fine.