Ancistrus sp. Rio Ucayali wears a splash of color on its fins, which are adorned with distinct rows of orange-to-rusty red spots. These orange-red fin spots are larger than and stand out against the silver-white flecks on the body, and it is these fin spots that give this pleco one of its trade names, the "orange spot bristlenose pleco." Unlike some Ancistrus, both male and female A. sp. Rio Ucayali possess bristles, so sexing young adults can be problematic. In adulthood, females have the typical wide midbody seen in other plecos when full of eggs; also, the females' bristles are smaller than the males', and restricted to the lips.
Ancistrus sp. Rio Ucayali can be confused with a number of other Ancistrus species, but can be told apart by distinct color differences and spotting patterns. Perhaps the closest match is Ancistrus hoplogenys. A. hoplogenys matches A. sp. Rio Ucayali in almost every regard except the spotting on the unpaired fins: in A. hoplogenys, the dorsal and caudal fin spots are much larger, rounded, less numerous, and not so clearly arranged in distinct rows, whereas in A. sp. Rio Ucayali, the spots are smaller, irregular-shaped, more numerous and aligned in distinct rows.
Several other Ancistrus also resemble A. sp. Rio Ucayali: Ancistrus dolichopterus, A. leucostictus, A. punctatus, A. sp. L071, and A. sp. L180 to name a few. A. dolichopterus can easily be distinguished (at least in smaller adults) by the white seams on its dorsal and caudal fins, and by the lack of the distinctive orange-red fin coloration; the other species listed have body spots similar to those of A. sp. Rio Ucayali, but lack the orange-red spots on the fins and may show no fin striping patterns whatsoever. L071 can be further distinguished from A. sp. Rio Ucayali because the spots on L071 are round instead of irregular shaped.