As terdon and FumbleFingers indicate in their comments, butyraceous comes from the Latin word butyrum (butter) and the suffix -aceous ("characterized by" or "of the nature of"). It appears in Samuel Johnson's dictionary of 1756, with the definition "Having the qualities of butter."
Google's Ngram Viewer shows an interesting graph of the term's usage, indicating that butyraceous appeared proportionately most frequently in the years between 1730 and and 1900. Of course in the 1700s, one occurrence constituted a much more significant proportion of total usage than it does today.
The first occurrence that Google Books finds from 1600 forward is in Peter Shaw, Philosophical Principles of Universal Chemistry (1730), a book in which the term appears four times:
But the Greatest part of Benjamin [a resin] sublimes in the form of Flowers; after which comes off also a thick or butyraceous kind of matter.
To this ball must again be added a little of the Mercury that was squeezed from it; or so much as to reduce it to the consistence of a somewhat soft and butyraceous mass;
Or else without such a previous digestion, the mixture is directly put into a small glass Retort, with a very wide neck, and slowly urged with proper degrees of Fire in a Sand-Furnace; by which means the acid Spirit of Salt contain'd in the Mercury-Sublimate, seizes the Regulus, imbibes, dissolves, and thence carries it over the helm with itself; and at the same time gives it a thick or butyraceous consistence, whence 'tis called by the name of Butter of Antimony.
The Production of this Operation is called philosophical Gold on account of its being so highly attenuated, subtilized, and brought to such an impalpable Powder, that when mix'd in this form along with fresh Mercury, 'tis immediately dissolved thereby; after the same manner, to illustrate the thing by a gross example, that Leaf-gold is more expeditiously and easily ground into a fine butyraceous amalgam with common Mercury, than such Gold as is only granulated or used in the form of coarser Filings.
The other six occurrences prior to 1764 that Google Books finds are likewise in scientific publications and appear in discussions of chemistry, botany, or medicine.
The first figurative or purely literary occurrence of butyraceous that I've been able to find in Google Books search results is from "Personal Recollections of the Late Mrs. Hemans," by H.F.C., in The Museum of Foreign Literature and Science, vol. 27 (1835):
I had such a levee yesterday morning, I was as much inclined to run away from them all, as from the Bishop and Dean, and sofa-table, and Chinese puzzles of old. ——— and ——— called upon me—what a butyraceous-looking pair they are!