What "suckers" are they talking about?

With Jake's execution scheduled for 0600 military time in Avatar Fire And Ash (2025), Na'vi warrior Neytiri steals an RDA security tag, using its tracking beacon to trick base defenses into seeing her as friendly. Disguised in Mangkwan clan paint to avoid alarms, she slips into the base while four soldiers chat nearby.

[soldier 1] Ah, it's not crazy, man. If I was four or five feet taller.

[soldier 2] Yeah. Felt the suckers, but they're sharp as blades too. They're not soft.

What "suckers" are they talking about?

They're almost certainly referring to the Southern Squidray. The soldiers have observed them being ridden and are macho-talking about how they could do that if they were as tall as a Na'vi. One of the soldiers has apparently seen a squidray in the wild (or more likely a corpse) and notes that the suckers themselves are razor-sharp, so it's not just a matter of getting on and holding on.

_{^{Avatar: Fire and Ash: The Visual Dictionary}}

The Southern Squidray (Na’vi: Tsyong) is a deep-water predator that shares many similarities with Terran squids. These efficient hunters typically dwell far below the surface, near hydrothermal vents where their primary sources of food also can be found in abundance. They are highly social and efficient hunters, using a variety of tactics to stalk and confuse prey. During hunts, they communicate with other members of their pack using bioluminescent chromatophores which contain millions of pigment cells. When “chattering”, they can give spectacular displays of color that confuse or disorient prey. These pigments can also be used for camouflage.

Unlike their Terran counterparts, the Tsyong only have two long tentacles, which separate into elongated digits with fingerlike tips. These tentacles are lined with suction grips to help seize and trap prey. The Tsyong also have two intakes and siphons on the side of their body. Coupled with its ability to streamline its body into a torpedo-like shape, these biological engines help propel the Tsyong at startling speeds, even known to be capable of launching them airborne.

_{^{Pandorapedia - Squidray}}