Discovering a Lump of Flesh can be frustrating in Elden Ring, mostly because it is not guaranteed to drop from enemies. The in-game item description says that it mainly comes from carnivorous beasts, but only two hold a high chance of carrying the flesh. Once picked up, the crafting material can be made into one powerful consumable.
Lumps of Flesh are most likely to drop from boars. Unfortunately, it is hard to find more than a few boars throughout the game, but there are two roaming in the fields north of the Church of Elleh (as marked below). Secondly, it is worth checking Limgrave and Caelid’s shorelines for a Lump of Flesh, as they typically spawn carcasses that carry the material. Dogs and wolves can also be pillaged for Lumps of Flesh, but the odds of them holding it are very slim.
This material is exclusively used to craft a consumable named Exalted Flesh. As one would expect from a protein, this item increases your physical attack stat, but the effect will last for a minute each time it is used. You can unlock its recipe by grabbing the Deserter’s Cookbook  located in the lone home set within Dragon-Burnt Ruins. After, you will be required to have five Rowa Fruits, one Hefty Beast Bone, one Arteria Leaf, and one Lump of Flesh to craft Exalted Flesh.