Manrent refers to a Scottish contract of the mid-15th century to the early 17th century, usually military in nature and involving Scottish clans.
In 1430 Clan Forbes entered into a bond with a branch of the Ogston family. This was described as follows:
10 May 1430; for Alexander Ogston of that Ilk, son and heir of Thomas of Ogistun lord of that Ilk, to "attend him [Sir Alexander Forbes of that Ilk] with three armed horsemen against all mortals, the King excepted"; with a clause that the number of horsemen be increased to six upon Thomas of Ogistun's death: "...sall serff wt. III hors qwyll my ffader lyffis & efter hym wt. sex"
Information on this can be found on Wikipedia here and in the book The Scottish Clans and Their Tartans which can be viewed online here.