Tentacruel is based of the real world Portuguese Man-of-war (a jellyfish like animal but not a true jellyfish) the Portuguese man-of-war has the ability to withdraw tenticals to increases speed or extend revile them to slowdown and catch more pray, even rip them off with no injury.
Tentacruel hides 66 out of 80 of its tenticals, no wonder its SPEED stat is so high.
Its good pokemon science. =)