Tauros
Typing: Normal -----> Normal
Abilities: Intimidate / Anger Point / Sheer Force ------> Sheer Force
75/100/95/40/70/110 -----> 75/150/105/60/85/115
New Moves: Head Charge
Tauros was king back in RBY. Back when there was no Steel-type, the only Ghost-type was Gengar, and all Normal-type moves were physical. However, with each passing generation, it appears to only be falling deeper and deeper into obscurity. Though it now resides in NU, however, it by no means is a poor Pokemon. Passable Attack, good Speed, and a strong ability all make Tauros a force to be reckoned with in the lower tiers; however, it simply doesn't have the strength to make its mark in OU, especially as the threats around it keep getting stronger while it remains mostly the same as it did over a decade ago. Mega Tauros brings Tauros back into the limelight as a powerful Normal-type physical sweeper.
Tauros keeps Sheer Force because it's a great ability that Tauros can abuse well. Rock Climb is its primary STAB move of choice; with Sheer Force, it has a BP of 117 that makes it stronger than Return. Head Charge is basically the same in power, but has recoil, and thus is more of a flavor addition than anything. It also remains Normal-type because I felt like a type change was unecessary, both competitively and flavorwise, for Tauros.
Tauros's Speed only increases 5 points, but that small increase puts it above the likes of the Latis, Tornadus and Thundurus-I, Gengar, and Mega Lucario. The main boost it gets goes to Attack. While 50 points may sounds like a lot, Sheer Force gave standard Tauros the ability to hold a Life Orb with no recoil damage; this meant that a Tauros with a Life Orb effectively had 130 Attack. Mega Tauros has more than this to definitively justify its use over standard Tauros, as the bulk increase isn't large enough to justify using one's Mega slot on Tauros alone. Speaking of bulk increases, Tauros's average Defense gets pushed just over the 100-mark, while its middling Special Defense is boosted up to a slightly better 85.
Make no mistake, Tauros is meant to be a physical attacker, and a strong one at that. Some damage calcs:
252+ Atk Life Orb Sheer Force Tauros Rock Climb vs. 252 HP / 184+ Def Gliscor: 196-231 (55.3 - 65.2%) -- 98% chance to 2HKO after Poison Heal
252+ Atk Life Orb Sheer Force Tauros Rock Climb vs. 252 HP / 252 Def Rotom-W: 230-270 (75.6 - 88.8%) -- guaranteed 2HKO after Leftovers recovery
Sheer Force Rock Climb off of a base 150 Attack stat
hurts, to say the least. But really, Mega Tauros is only a bit stronger than standard Tauros with a Life Orb. If it can hit so hard, what's holding standard Tauros back from the higher tiers? Ghosts. Defensive Rocks and Steels. Anything that can outspeed or has moderately strong priority. In fact, it's these same things that keep Mega Tauros in check as well. Both Skarmory and Mega Tyranitar, for example, put a stop to Tauros, while Gengar can murder it with a Focus Blast since its immune to its strongest attack (it has to watch out for Zen Headbutt, but it can manage, particuarly if it has a Speed-boosting nature). Azumarill can take a healthy Mega Tauros out with Superpower, or a weakened one with Aqua Jet. While Mega Tauros may be fast and strong, it certainly isn't unstoppable when compared to other threats in OU.
Mega Tauros is a fast physical attacker, the same role that he's fulfilled for over 15 years. However, thanks to his new stat increases, he can now fulfill this role even in the higher tiers, just as he had back in RBY. In 1998, Tauros was a powerful threat in the metagame, and now Mega Tauros can be the same.