From The Collaborative International Dictionary of English v.0.48:
Bots \Bots\, n. pl. [Cf. Gael. botus belly worm, boiteag
maggot.] (Zool.)
The larv[ae] of several species of botfly, especially those
larv[ae] which infest the stomach, throat, or intestines of
the horse, and are supposed to be the cause of various
ailments. [Written also botts.]
Note: See Illust. of Botfly.
[1913 Webster]