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In the U.S. phrases (as) regular as pig tracks and (as) common as pig tracks, the plural noun pig tracks is an intensifier.

Those phrases seem to have originated, and to be chiefly used, in the Southern United States.

The phrase (as) common as pig tracks occurs, for example, in Whither the noble quail?, by Jim Casada, published in The Herald (Rock Hill, South Carolina, USA) of Sunday 15^th November 2015 [page 8C, column 3]:

Raptors are protected by federal law, and as anyone who pays much attention to power lines and barren trees in winter surely must realize, once endangered hawks are as common as pig tracks.

These are, in chronological order, the earliest occurrences of the phrases (as) regular as pig tracks and (as) common as pig tracks that I have found:

1-: From The Flush Times of Alabama and Mississippi. A Series of Sketches (New York: D. Appleton and Company, 1853), by the U.S. attorney and author Joseph Glover Baldwin (1815-1864) [John Stout Esq., and Mark Sullivan, page 307]:

“They can’t do nothing with me: it was done fair,—it was an old quarrel. We settled it in the old way: I had my rifle, and I plugged him fust—he might a knowed I would. It was devil take the hindmost. It wasn’t my fault he didn’t draw trigger fust—they can’t hurt me for it. […]”
“Ah, Mark,” said John, “I aint so certain about that; that is, unless you are particular well defended. You see, Mark, it aint now like it used to be in the good old times. They are getting new notions now-a-days. Since the penitentiary has been built, they are got quare ways of doing things,—they are sending gentlemen there reg’lar as pig-tracks. I believe they do it just because they’ve got an idea it helps to pay taxes. When it used to be neck or nothin’, why, one of the young hands could clear a man; but now it takes the best sort of testimony, and the smartest sort of lawyers in the market, to get a friend clear.”

2-: From Mining News, published in the Columbia Gazette (Columbia, California, USA) of Saturday 24^th June 1854 [page 2, column 3]:

The claims on Matelot Gulch, above the Quartz mill are paying from $10 to $12 per day, as regular as Pigs’ tracks.

3-: From Who Will be our Governor?, published in the Edgefield Advertiser (Edgefield, South Carolina, USA) of Wednesday 6^th December 1854 [page 2, column 4]:

Undue influences to secure votes, one way or another, are as common now-a-days as pig tracks. And what difference does it make whether it be done by buying votes openly or purloining them secretly.

4-: From the Wetumpka Spectator (Wetumpka, Alabama, USA) of Thursday 22^nd January 1857 [page 2, column 2]:

One of the Boats we write about.—The Coosa Belle left our wharf on her last trip with twenty-one cabin, and forty-two deck passengers, and eight hundred and fourteen bales cotton.
That’s right Wetumpka, stick to the “Belle,” and next summer, when that old rock down near the wharf begins to show its head above the water, it won’t frighten Capt. Olds, Capt. Peter, or any body else on that craft. But whenever there is water enough to float a terrapin, just as regular as pig tracks the “Belle” will be at our wharf, and Peter will be on our streets, with a book under his arm and a smile on his face.

5-: From the Yorkville Enquirer (Yorkville, South Carolina, USA) of Thursday 8^th December 1859 [page 1, column 5]—reprinted from the Charlotte Bulletin (Charlotte, North Carolina, USA):

We once published a paper in South Carolina, which was issued weekly, and in order to obtain the latest news we subscribed to several daily papers for our special benefit; and others equally valuable, as a favor, were sent to us in exchange. Soon after commencing business we were terribly bored with a “sucker” in the neighborhood, who called in “as regular as pig tracks,” directly after the mails were obtained, to see the morning papers of a neighboring city, to learn the price of Cotton, Corn, &c.,—the reports of the previous days transactions,—as well as the latest news from Europe, but he would not subscribe for the paper because he could get the reading of it for nothing, as he said.

6-: From The Yazoo Democrat (Yazoo City, Mississippi, USA) of Saturday 23^rd June 1860 [page 3, column 1]:

Eclectic Magazine.—This popular monthly is as regular as pig tracks. The July number, which is now before us, is filled with gems of European literature, from the pens of the most eminent writers of that country [sic]. It is embellished with a portrait of the late Governor-General of Canada, Lord Elgin.

7-: From the Georgia Journal and Messenger (Macon, Georgia, USA) of Wednesday 11^th July 1860 [page 2, column 7]:

No Douglas Men.

The Athens Watchman thus speaks of the cry raised by the seceders some time since in the 6^th District:
We are daily told that there are no Douglas men in the Sixth District! What a prodigious—mistake! They are “common as pig tracks,” even here in Athens.