Horseshoe crabs (order Xiphosura, "sword-tail"), which date back at least 450 million years, are older than:

- crabs* (~200 million years ago)
- horses (~50 mya, tops)
- horseshoes (~1000 years ago, tops)
- swords (the _xiphos_ is Iron Age; ~1200 ya)

Not tails though.

* that's right, horseshoe crabs are not crabs. They are more closely related to arachnids and, according to some recent work, may even be more closely related to some arachnids than some arachnids are to each other, which you could think of as "horseshoe crabs are arachnids" but which really means "the group 'arachnids' isn't a true group and we need to make a new category." However, other work supports the traditional "family tree" with them solidly outside arachnids. At any rate, they are in the branch of arthropods called Chelicerata (also including arachnids and sea spiders) rather than Mandibulata (crustaceans, insects, etc.; all the other arthropods.)