Hypoptopoma cf. gulare. Like their smaller and more popular cousins, the Otocinclus catfish, these are schooling suckermouthed grazers. They do best in mature aquaria that are stable and have lots of algae/biofilm to feed off of. Giant Otos are still small for Loricariids (the armoured suckermouth catfish family) reaching only about 4", compared to the 2+ feet some plecos can attain.
These are young adults around 3" long.