Strange and/or Massive
Here are some of the largest and oddest beasts of the group.
(Mammals with a * are featured on the BBC's "Walking with Beasts")

Indricotherium transsouralicum* Height: 16ft (4.7m) Lived: 30 million years ago - 25 million years ago   Also known as Baluchitherium or Paraceratherium. Indricotheres were   the largest mammals to ever walk the earth. Depending on the source, these   overgrown rhinos were thought to weigh between 11 and 20 tons. Links: [ 1, 2, 3, 4, 5, 6, 7, 8 ] Chalicotherium* Height: 9ft (2.75m) Lived: 45 million years ago - 3.5 million years ago   Strangest of the bunch, Chalicotheres have the shortened face of a horse,   the neck of an ox, the arms and shoulders of a massive ape, the front claws   of an anteater and short rear legs to support its massive bulk. Links: [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] Gigantopithecus blacki Height: 10ft (3m) Lived: 700,000 years ago - 125,000 years ago   Largely based on fossil teeth found throughout China, Gigantopithecus was   estimated to be over 10 feet tall. Also possibly the source of many stories   about Yetis, Sasquatch and other massive "Ape-Men". Links: [ 1, 2, 3, 4] Elasmotherium sibiricus Height: 6.5ft (2m) Lived: 1 million years ago - 10,000 years ago   Sporting a horn well over 6 feet long, Elasmotherium may have been a part   of the inspiration for Unicorns. Of course beauty doesn't seem to be factored   here (much like the story of manatees inspiring the mermaid myth.) Links: [ 1, 2 ] Doedicurus (Glyptodon)* Height: 5ft (1.5m) Lived: 1.5 million years ago - 12,000 years ago   Part Armadillo and part Ankylosaurus, this small volkswagen of a creature   roamed over most of South America before the last Ice Age. Links: [ 1, 2, 3, 4, 5, 6, 7, 8 ] Uintatherium robustum Height: 5ft (1.5m) Lived: 50 million years ago - 35 million years ago   With a hatrack for a skull, this large herbivore is distinctive, yet still not very   well understood. Uintatherium had tusks, two pairs of knobs and a set of horns   protruding from its head, with a rather large but nondescript body to support it. Links: [ 1, 2, 3, 4, 5, 6 ] Odobenocetops peruvianus, or Walrus-Whale Length: 7ft (2.1m) Lived: 5 million years ago - 1 million years ago   An odd whale, with assymetric tusks. Slightly larger than a modern dolphin,   Odobenecetops developed much like a walrus - equipped to feed on sea bottoms   and rocks. Links: [ 1, 2, 3 ]