If you accept far-fetched argument, I've got an individual that weight more than a ton! But let's start with a simple example of a large troglodyte.
Blind cave eel
The blind cave eel (Ophisternon candidum) is a troglodite. It is a pure white fish growing to 40 cm, with an eel-like body, no eyes, and a thin rayless membrane around the tip of the tail. I could not find its weight but it probably does not weight much! Just like many troglodite species, its distribution range is very restricted. It only occurs in a single location in Western Australia.
Well a fungus is not an animal (but is closely related), so it does not answer your question but I still wanted to mention it. There's an individual fungus that covers 965 hectares. See this popular article.
Ant supercolonial individual
The individual ant is of course not that big but it is not uncommon to consider a colony as a superorganism. You might want to have a look at major transitions in evolution: revisited for a discussion about the concept of individual for social species. Note also that most ant casts spend most of their time underground in the nest.
Until 2000, the largest known ant supercolony was on the Ishikari coast of Hokkaidō, Japan. The colony was estimated to contain 306 million worker ants and one million queen ants living in 45,000 nests interconnected by underground passages over an area of 2.7 km2 (670 acres). In 2000, an enormous supercolony of Argentine ants was found in Southern Europe (report published in 2002). Of 33 ant populations tested along the 6,004-kilometre (3,731 mi) stretch along the Mediterranean and Atlantic coasts in Southern Europe, 30 belonged to one supercolony with estimated millions of nests and billions of workers, interspersed with three populations of another supercolony. The researchers claim that this case of unicoloniality cannot be explained by loss of their genetic diversity due to the genetic bottleneck of the imported ants. In 2009, it was demonstrated that the largest Japanese, Californian and European Argentine ant supercolonies were in fact part of a single global "megacolony".
Considering a weight of 3.5 mg per individual, a colony of 306 millions ant weight more than a ton (1071 kg exactly) and of course this excludes their constructed habitat which could arguably be considered as part of the individual.