Unicorns are sentient, good-natured horned horses with some innate protection against evil. The celestial charger variant is a unicorn with the celestial creature lens applied to it. In the Monster Manual it also has cleric levels, but clerical magic in my setting is not very compatible with the unicorns, because they must be able to manipulate their patron deity's holy symbol.
Troglodytes are underground-dwelling reptilian humanoids that can exude a nauseating musk. The racial template is cheap enough to be fitting for player characters. I've always been fond of the troglodytes.
Tritons are semi-aquatic beings that migrated to the Material Plane from the Elemental Plane of Water. Why are they able to breathe air and walk on land (although clumsily) if these abilities are not requires on the Elemental Plane of Water?
Treants are animated trees that can animate other trees. Not much else to say.
Tojanidas are weird turtle-crab-squid hybrids from the Elemental Plane of Water who like talking about food.
Titans are chaotic outsiders of great size, strength, and intelligence. Their array of spell-like abilities is quite diverse.
The tendriculos is a carnivorous slitering plant monster that swallows prey and paralyzes it with its corrosive digestive juices. That's a strange one.