See "Gladiator Beast Alexander" for additional rulings.
If “Gladiator Beast Hoplomus” is Special Summoned by the effect of a “Gladiator Beast” monster, its original DEF becomes 2400. If “Skill Drain” is later activated, this effect is negated and “Gladiator Beast Hoplomus’” original DEF will return to 2100. Even if “Skill Drain” is later negated or removed from the field, the original DEF of “Gladiator Beast Hoplomus” will remain at 2100.
If “Gladiator Beast Hoplomus” is Special Summoned by the effect of a “Gladiator Beast” monster and it is flipped face-down, when it is flipped face-up its effect will not be reapplied so its original DEF will be 2100.