Fensir are a race of trollkin native to the plane of Ysgard, earning them the moniker "Ysgardian Trolls". They are based on Nordic mythology, and their name is a corruption of one of the Nordic words for "Troll". While they share something of the rapacious appetite of material plane trolls, they are vastly more cultured, intelligent and civilized, and generally just want to be left alone, although they occasionally trade with other races and some fensir have been known to seek human mates.
The one exception to their usual reclusive amiability is when a female fensir responds poorly to giving birth and transforms into a Rakka; an insatiably hungry giantess who becomes consumed with the need to devour everything she can get her hands on, and which her family will respond to by stripping the countryside bare to feed her until she finally dies within a few years. In AD&D, this happened to every female fensir once she had given birth for the first time; in 3e, realizing that this was stupid, they instead changed the Rakka transformation into something that only happens occasionally, and only after the first birth; subsequent births by an untransformed fensir woman don't risk changing her.
Fensir have something of an ongoing feud with lillends. It's left ambiguous if this is because male fensir have a tendency to fall in love with the beautiful angel-lamias or if it's because they believe lillend blood is needed to brew the medicine that protects them from being turned to stone by sunlight (or the one that cures it).
Fensirs debuted in the Planescape boxed set "Planes of Chaos" for Advanced Dungeons & Dragons 2nd edition, before being updated to Dungeons & Dragons 3rd Edition in the Fiend Folio. They haven't been seen since. They are one of several planar races that dedicated Planescape fans converted into a PC race, and they can still be found online at the Mimir website.