I sometimes see these words used interchangeably.

Problem:
The words “hoard” and “horde” are not synonyms.

Explanation:
The word “hoard” — spelled H-O-A-R-D — dates back to before 900 to the Gothic word “huzd”, which literally meant treasure.

When it acts as a noun, the word “hoard” means an accumulation that is carefully guarded for future use.

When it acts as a verb with an object, the word “hoard” means to accumulate for future use in a hidden place.

When it acts as a verb without an object, the word “hoard” means to accumulate food, money, or anything else valuable in a hidden place for future use.

The word “horde” — spelled H-O-R-D-E — dates back to about 1550 but apparently originated before then with the Turkic word “ordu”, which literally means royal camp or residence.

The primary meaning of the word “horde” as a noun is a mass or crowd, and an alternate meaning is a nomadic group.

The word “horde” can also be used as a verb without an object, in which case it means to gather or assemble in a horde.

Solution:
Remember that the word “horde” — spelled H-O-R-D-E — seems to come from the Turkic word “ordu”, which means royal camp or residence, to remember that this word relates to a crowd or mass or group of people and to distinguish it from the word “hoard” — spelled H-O-A-R-D — which comes from a Gothic word for treasure.