equisatum, with an a in the middle

That e is the whole of the second half of the name. Seta is the Latin word for a bristle or a stiff hair, and it is still in use in biology for exactly that: the bristles on an earthworm are its setae. The horsetail is named for the ring of thin branches at each joint, which stand out from the stem like the hairs of a brush. Write sata and the bristle is gone.

An unstressed vowel, flattened until it gives the ear no information.