A teaser is a hormonally active but sexually incompetent male sheep. Usually a 'rig', where one testicle hasn't descended - either through nature, or more usually castration leaving one testicle inside.
Winter before last we had a really cold spell mid-December. Mid-May a number of us suddenly had unexpected lambs, or incorrectly bred lambs. It turned out that it was so cold that the internal testicles of teasers running with ewes began to work!
A neighbour uses Blue-faced Leicester teasers then leaves them running with the mule ewes. Mid-May he had a batch of lambs from the mules that were certainly not from Texel tups.
We had accidentally
left a teaser running with the keeping ewe lambs (not supposed to be tupped) and had 10 of them lamb unexpectedly mid-May.
You can be sure that this winter, all teasers were carefully removed from ewes as soon as the cold weather came! (And on this farm we double-checked the sex of all our keeping ewe lambs ...)