A creature brews any fleshgrafting poison at the GM's discretion
The feat Brew Fleshcrafting Poison comes from Endless Night, the fourth chapter of the Second Darkness adventure path for Dungeons and Dragons 3.5 (that's what OGL after the path's entry on the Paizo Web site means). The feat's included on the d20PFSRD site, but it's unavailable on Paizo's SRD. Ask the GM whether material that was intended for D&D 3.5 (which is nominally Pathfinder compatible) is allowed in the campaign.
If the feat is allowed, one should then consult Endless Night and its (presumably closed content) descriptions of fleshgrafting poisons: antennae, blinding spittle, burrowing claws, chitinous skin, poison fangs, retractable spines, and stinging tail. (Tip: Some are okay.) One should also keep in mind that as of this writing there have been no additional fleshgrafting poisons in any other product.
Those fleshgrafting poisons that are described are specific yet don't mention actual creatures (for example, the antennae says, "You sprout antennae from your eye sockets, gaining blindsight out to a range of 30 ft. and normal (human-level) vision" (63) rather than, for example, "You sprout antennae like a rust monster"). Further, each poison has a corresponding penalty while the poison's in effect (for example, for the antennae the penalty is −2 hp per HD). (They are, after all, feshcrafting poisons.)
In short, trying to divine how to use the feat Brew Fleshgrafting Poison solely from the information provided by the d20PFSRD is a frustrating and ultimately fruitless exercise.
Thus, for example, were I the GM I would look askance at a player who tried to convince me, as posited by this Reddit thread, that a creature can use the feat Brew Fleshgrafting Poison to create a magic item costing 2,000 gp that forever grants the imbiber the dwoemercat cub's supernatural ability dweomer leap... especially if the player hadn't consulted the feat's source (which describes in greater detail the effects of chugging a fleshcrafting poison) and argues that there should be no side effect (as each and every printed fleshgrafting poison has one).
In other words, like some other niche item creation feats, the player should first have the full text of the feat—including the items the game already says the creator can create using the feat—and know how those items function, then the player can consult with the GM about adding new items to the extant list. (Then the creator and the GM can thumb wrestle over pricing by first comparing the new item to similar existing items, as always.)