1. A large number, mass, crowd, multitude, group, etc.
2. A tribe or group of Asian nomads.
3. Any nomadic group of people or animals.
4. To gather into a large number.
EX. Let me start by saying... I don't like horde battles.
Ideally, in any rpg, when you start out fighting weaker enemies, moving up the ranks of difficulty, until you can take on more and more difficult challenges with each step forward. In a very simple and typical Dungeons and Dragons adventure you'd expect to see a low level party take on, you might start by fighting a small group of goblins, then usually a bigger monster or two like a couple bugbears, then you fight a boss monster like a young dragon they'd been keeping as a pet. That is the progression you wanna see in an RPG. As an added bonus, you have those players fight not just a bunch but a small horde of goblins. Unlike when they first fought they goblins, they now have the tools to properly take them on and, if they use them properly, can dispose of the great number of foes with ease. They can compare their two experiences with goblins and feel as if they have grown tremendously by realizing how they've gone from struggling with a small group of goblins to dominating a small army of them. In 4th Edition, they specifically created minions to help DMs construct this power trip of an experience.
Pokémon is an RPG too. You're supposed to feel that you're team has grown so that, by the time you're the champ, you can believe it. Ideally, horde battles should be challenging but give you the opportunity to feel powerful. While your Chespin once struggled to knock out a Pidgey at the beginning of your journey, your Quilladin should be able to easily handle a gang of Scraggy by the time you run into them but, instead, that is not the case. Rather than taking out several Pokémon at once, you have to usually take them out one at a time while they get five hits on your Pokémon. Rather than feeling like your knocking them out with your raw power, you're instead slogging through several turns to get the EXP and/or catch the last conscious Pokémon. It doesn't feel rewarding because a) there are not many moves that hit multiple opponents and b) it is a tedious affair if and especially if you wish to catch one of the members of the horde.
In the end, what could've been a satisfying part of the experience comes off a frustratingly mediocre addition that may be the weakest type of battle ever introduce.
Think about how satisfying it would be to be able to target multiple targets like, let's say, a horde of Zubats, with Thunder and wipe them off the screen? Maybe you could get rewarded based on your performance? What do you think?