A horse is a creature just as a player character is (see this Q&A). Thus, a horse can benefit from goodberry with no issues.
If the spell was intended to be restricted to certain creature types (humanoids, for example) the spell would say so.
It is worth noting that goodberry is apparently not intended to provide water according to Jeremy Crawford:
The nourishment provided by goodberry is meant to be like food, not water, but a DM can say otherwise.
Side point: Carrying food is easier than you think it is
This is not something you directly asked about, but you may find it useful to know that you may be making feeding you mount into a bigger issue than it needs to be. While using a single 1st level spell slot a day to feed horses (and yourself) is not a huge deal, it does require you to have that slot available to use, which may not always be convenient. Consider three points:
The rules say that 1 day's worth of food for your mount weighs 10 lbs (PHB) (not 20 as in your question).
The carrying capacity for a riding horse is 480 lbs which should be a good amount to carry food for it as well as you and or your gear. Even if a riding character and their gear weighed 250 lbs. that still leaves at least 230 lbs for other things. If you used all that extra space for food it would be 23 10 lb rations of food. Enough to keep one horse fed for 23 days or 4 horses fed for about 6 days.
This food would most likely be stored in the horse's saddlebags. Unfortunately, there are no rules that I can find that specify how much capacity the saddlebags have, so your DM will have to help you there. But it seems very reasonable that they would be capable of storing at least two day's worth of food. See this Q&A and this one for more discussion/ideas on the matter.
Don't forget that horses can graze on grass and other vegetation as part of their food intake. This may be able to reduce the amount of food you are required to bring for it without resorting to using spell slots.