For a start, it seems to have existed as a thing for longer than India Pale Ale, a term to which the first known references only date from the 1820s. Pale Ale was exported to India earlier than that date, it's true, seemingly before the term India Pale Ale was coined.
My oldest East India Porter recipe - brewed originally by old friends Barclay Perkins - dates from 1805. But the term has been around longer than that. As this proves:
"ENGLISH AND IRISH ALES, INDIA PORTER, &c.
T PIDGEON, No. 30, Cusse-street, respectfully informs the Public, that his stores are constantly stocked with the following articles, of genuine quality and prime order:
It's interesting to note the price differential between ordinary London Porter and East India Porter: 45.5d to 65d. That's around 50% more. But I know from the 1805 Barclay Perkins records that the gravities of the two types of Porter were the same, the only difference being hopping. The India version had around double the hops of the domestic one.
Per Doz. Per Doz. Bellingham Ale, 7 7 East India Porter, 5 5 Windsor do. 5 5 English do. 3 9h Fermoy do. 5 11h Andrews’s Pale Butt 5 11h Appledore do. 5 5 Double Spruce Beer, 2 8h
Four dozen English Porter, at 3s. 6d. per dozen.
Wines usual of best quality, for ready money only."
Saunders's News-Letter - Thursday 08 March 1804, page 2.
India Porter was already around in the 19th century:
"TEA WAREHOUSEThat's a slightly confusing advert. Were they shipping the Porter back from India?
WADE and MARSHALL, of the Earl Macartney indiaman, 16, Dame-street, have received from the east India Sales, per the Suir and Eliza, from London, fresh of Teas, Sugars, Spices, and other articles, which are of the finest flavour and quality, the lowest terms for ready money. They have also supplied themselves with a quantity Dr. SOLANDER’s SANATIVE ENGLISH TEA, which has been so highly improved his experience, acquired in his voyage round the world with the celebrated Captain Cooke, and which is approved and recommended by physicians of the first eminence in England, the most pleasing and powerful restorative in all Nervous Disorders. Their Family WHISKEY is remarkably fine flavoured, old, and well worth the attention of private families —very fine old Claret — Port — White Wines — and East India Porter, in wood and bottle — with all kinds of foreign Spirits. N. B. Wholesale purchasers will meet with encouragement, and find it their intercit to apply."
Dublin Evening Post - Thursday 09 March 1797, page 3.
And here's the oldest reference that I've found so far:
"East India Porter, in high order 8s. 6d. per Doz. BEST LONDON PORTER, in Wood and Bottle.Not many details there, unfortunately.
Town and Country Retailers supplied the most liberal Terms, WILSON’S WINE-VAULTS and London Porter-Stores, No. 13, GRAFTON-STREET, Dublin."
Dublin Evening Post - Thursday 19 January 1792, page 3.
My guess is that the term East India Porter is older than 1792. The way it's referred to implies a consumer would know what it was.