Well Mantine and Remoraid and not fixed together much like Slowbro or Slowking are when a Shellder attaches it's self to a Slowpoke. Remoraid just clings to Mantine's body to scavenge it's Leftovers so it isn't actually part of it's body.
Generally, it is pictured with a Remoraid attached to its underside, much like a cleaner fish.
It tends to travel by attaching to Mantine's fins or in large groups of its own kind. When attached to a Mantine, it feeds on whatever the Mantine leaves behind.
The reason why Remoraid is no longer seen under the fin of Mantine is probably because Remoraid doesn't disappear when Mantine evolves from Mantyke.
Despite the fact that Mantine evolves from Mantyke in the presence of a Remoraid, Remoraid is not present on any of Mantine's sprites in the generation the evolution method was introduced nor in the newer generation. This may be due to fact that the Remoraid used for evolution doesn't disappear after Mantine evolves from Mantyke.
Mantine has been seen with and without a Remoraid attached to it in the Anime:
Despite Remoraid not being attached to Mantine's fin in Mantine's Generation IV-VI sprites, it is still acknowledged that they travel together in their Pokédex enteries:
Mantine
Pearl: It swims elegantly, mindless of Remoraid hitching on to its broad fins. It has a docile nature.
Platinum: While elegantly swimming in the sea, it ignores Remoraid that cling to its fins seeking food scraps.
HeartGold: As it majestically swims, it doesn't care if Remoraid attach to it for scavenging its leftovers.
Black/White: While elegantly swimming in the sea, it ignores Remoraid that cling to its fins seeking food scraps.
Y: While elegantly swimming in the sea, it ignores Remoraid that cling to its fins seeking food scraps.
Remoraid
Pearl: It clings to Mantine to feed on the big Pokémon's scraps. This is an adaptation to avoid foes.
Platinum: It forcefully squirts water. The water jet never misses prey even if the Remoraid is deep in the sea.
SoulSilver: Using its dorsal fin as a suction pad, it clings to a Mantine's underside to scavenge for leftovers.
Black/White: It forcefully squirts water. The water jet never misses prey even if the Remoraid is deep in the sea.
Y: Using its dorsal fin as a suction pad, it clings to a Mantine's underside to scavenge for leftovers.
Sources: Mantine, Remoraid
Hope I helped :)