That part of a watch case to which the strap is attached. Horns come in various shapes and are attached to the case middle. - https://www.hautehorlogerie.org/en/encyclopaedia/glossary-of...

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Modern Horn with Spring or Fixed Bar Strap Fixings

After the early fixed wire lug watches, more modern watches have "horns", projections fixed to side of the case, and "bars", either spring bars or fixed bars which engage with these horns. The bars usually come closer to the case than fixed wire lugs, and often they either don't leave enough space for a one piece leather strap to pass between the bars and the case. Also the placement of the bars on the mid line of the case often doesn't allow a one piece strap to wrap easily around the bars and then across the back of the case. The straps are wider and a back pad is not needed. - http://www.vintagewatchstraps.com/strapdetails.php

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You only have to loosen the two secured axis by pulling out the strap-mounting screws of the horns on the wristwatch, remove the bracelet. Put the new strap with the openings between the horns, slide the mounting bolts through the openings and fix it with the bolt screws on the opposite side. - http://www.feldo-luxury.com/wristwatch-v1.html