jungi

One of the best etymologies in the language, and it is free. Latin iungere means to join. The ox-yoke is the joining device, so the same root sits inside English yoke and English join, inside junction and conjunction and conjugal, inside Greek zygon (a yoke, hence zygote) and inside Sanskrit yoga - which is literally a yoking, the discipline that harnesses the self. Jungi is that verb, arriving in Esperanto with its farm meaning intact. If you can remember that yoga and junction are the same word, you can remember jungi.

Register: this is a working word from a working world. You jungi a ĉevalo to a kaleŝo, a pair of bovoj to a plough, a dog to a sledge. The thing being harnessed takes the direct object and the vehicle takes al: jungi la ĉevalon al la veturilo. It is transitive, so it has a natural -iĝ partner (jungiĝi, to be harnessed) and the useful opposite maljungi, to unhitch, which is what you do at the end of the day.

Two derived words you will meet. Jungilaro is the harness itself - the same -il- plus -ar- stack that gave manĝilaro two words earlier in this band, so a harness is the joining-tools-collection. And jugo, the yoke as an object, is a separate root from the same Latin family; it carries the figurative weight in Esperanto that English gives to the yoke of oppression.

The modern extension is alive. As in English, you can harness things that are not animals: jungi la venton, jungi la forton de la rivero. That is a nineteenth-century metaphor in both languages and it has aged well, which is lucky, because otherwise this root would have retired with the horses.