The Korean fantasy MMORPG Bless boasts 400 mounts. There is also a lot to offer in terms of pets and collections.
MMORPGs are paradises for collectors and completists. These are people whose left eye only stops twitching when they have every mount in white, black, spotted, striped, brown, and some exotic color (usually purple). The differently colored mounts are all equally fast, and no one really needs this … but that doesn’t matter at all for the passionate collector.
The MMORPGs know these desires of the players and are happy to accommodate them, though to varying degrees. While some games feature a manageable number of collectible items, others really go all out with the motto “more is more”.
Among the MMORPGs with an exceptionally rich offering is likely Bless. They promise over 400 mounts, which probably comes together since you are supposed to tame monsters as mounts in Bless. Additionally: collections, crafting, travel companions, and pets.

