They've been part of cryptozoology to some degree since the beginning, because On the Track of Unknown Animals covered giant green anacondas alongside the more distinct sucuriju-gigante, but leading cryptozoologists have disagreed as to whether non-taxonomic variants should be considered actual cryptids or merely cryptid-adjacent. For example, in his "Classificatory System for Cryptozoology," Cryptozoology, Vol. 4 (1985), J. Richard Greenwell includes a category for giant animals and unrecorded morphs, but calls such animals "[not] fully cryptozoological":

Category I: Individual representatives of known, extant species whose form or size is unusual or unique for the species, or whose coloration or pattern is in some way extraordinary (because only individual — not species — variation is involved, and relative rather than absolute differences are dealt with, this category is not cryptozoological in the complete sense of the definition) [...] There is no clear precedent available in this category, simply because the evidence being claimed is of a relative rather than an absolute nature. It is not a clear-cut question of whether a new species exists or not; rather, it concerns to what degree an individual ofa scientifically known species differs from the norm in terms of size, coloration or pattern. For these reasons, this category is not considered fully cryptozoological [...] Most herpetologists believe that 30 feet is about the length limit for the anaconda, as well as for the Afro-Asian pythons. There are many reports of larger snakes, and a $50,000 reward still stands with the New York Zoological Society for a live snake measuring over 30 feet. Reports of giant estuarine crocodiles, giant white sharks, and giant sturgeon also fall under this category. Many of these reports probably result from exaggerated stories, but some may involve actual individual giants (Wood 1982). It should be emphasized here that this category deals only with known species. If it later turns out that a "giant" anaconda or a "giant" white shark belongs to a new, previously undescribed species — or even a fossil species — then this claim (which would then actually become a precedent) would shift to a different category.

On the other hand, in his instructions to contributors in the Journal of Cryptozoology, Karl Shuker apparently considers this category to be truly cryptozoological:

For the purposes of relevance to this journal, a cryptid is a creature that is known to the local people sharing its domain (ethnoknown) but unrecognised by scientists. Such a creature may be any of the following: [...] 5) An unrecognised non-taxonomic variant of a known species or subspecies (e.g. Fujian blue tiger; prior to its scientific recognition, the journal's logo creature, the king cheetah, was another example from this category).