The group has spent up large at auctions in New Zealand, including $1.8m on 26 horses at Karaka in 2015.
A photograph on Lang's Rider Group website shows the businessman meeting Chinese President Xi Jinping and then Prime Minister John Key at Karaka in 2014.
In an interview last year, Lang said the affordability and quality of horses in New Zealand was the main reason he invested here, with efficient quarantine laws also a significant factor.
The company has worked with Auckland Tourism, Events and Economic Development (Ateed), and is a finalist in this year's Export Awards.
Labour will be happy with its own donation, having made fundraising a priority after National raked in four times more in donations in 2014.
Smellie gave Labour $100,000 on May 23, on top of the $15,000 he gave in December. He and representatives for Inner Mongolia Rider Horse group could not be reached for comment yesterday.
Labour Party president Nigel Haworth, who spends three or four days a week working on fundraising, said Smellie's donation was "extraordinarily generous" and the party was "humbled and delighted".
"I know he is impressed with where we are sitting in terms of policy. We have spoken to him, as we do to many people over the past couple of years, and he has been sufficiently impressed to make this extraordinarily generous offer."
Labour leader Andrew Little said the party had raised considerably more than at the same point before the 2011 and 2014 elections.
"We get a lot more small contributions and a lot online. And we are doing very well in that regard. In terms of the war chest, it is way ahead from where it has been at least in the last two elections and possibly more."
Haworth this year launched his "President's Club" for donors, which has various tiers of donor - bronze, silver, gold and platinum - who give certain amounts each year.
Asked if Smellie was a member, Haworth said Labour would make him one: "I think that would be the least we could do."