Jaya Ballard requires mana and a discard, but you can activate any ability immediately without worrying about counters
Iridescent Tiger's ability triggers if you cast it from any zone
Its vulnerable to powerful counterplay through cards like Bojuka Bog or Endurance, but it also has a lot of advantages
Other creatures I like but can't find space for are Skullmulcher , a creature + sac outlet and card draw on a body, Green Sun's Zenith or Chord of Calling this guy out when your sac outlets have been removed or countered to finish of a player second main phase, remember it is a may ability so can still keep enough tokens out for defense if needs be
For the main monster engine we have 3 Traptrix Myrmeleo, 3 Traptrix Pudica, 3 Traptrix Aracnocampa, 1 Traptrix Vesiculo, 3 Traptrix Mantis and 1 Traptrix Nepenthes