Yegani - Original Star Trek Species Profile

Behaviour

Yeganis are very complex, social, impulsive, instinctive, primal and emotional creatures, similar to humans in this respect, the differences being that they are alot more animalistic. This can be seen in their body language, vocalisations, language, psychology, etc.

for starters, they live in large multi-familial communities and all help each other out (7-9 families per settlement on average) with very strong social bonds and connections. instead of having one or two rulers in a given settlement, the settlement is ruled by the 2 oldest family members of each family (eg. a settlement of 10 families would have 20 rulers)

they communicate in a language known as "Yegani" which is the same level of articulation as the many dialects of earth. they also communicate in animal-like vocalisations such as growling, hissing, screeching, purring, yelping, etc. Yegani sounds like a mix of Arabic, Afrikaans, Samoan, and Urdu, combined with the clicks, whistles, trills and other animalistic sounds yeganis produce. There are two types of the language; Traditional, which incorporates the unique vocalisations that Yeganis make that are physically impossible for other species, and Simplified, which replaces these sounds with easier ones or removes them entirely.

They are mostly friendly to outsiders or anything or anyone considered different and or  unthreatening to them, because of their strong emotional connection abilities and large capacity for empathy. 

their usual combat style is wild and unpredictable. their displays of aggression are similar to that of apex predators, with bearing teeth and claws, adopting an aggressive stance/posture and producing a variety of vocalisations such as growling, hissing, and screeching. when they are in a combat situation, high levels of adrenaline are pumped into their bloodstream, making them resistant to damage and remarkably stronger than before, making them formidable opponents in combat.

because of their social nature, they have been known to tame animals on their planet and incorporate them into their large family groups.

behaviour changes during heat include, but are not limited to; greatly heightened libido, more aggressive and unpredictable, more passionate and affectionate, even more animalistic/crazed/manic behaviour, a voracious hunger to mate, hyperactivity, feral outbursts, etc. 

in the wild, they live primarily around or in water or around or in trees, hence their long muscular tails, used for swimming and climbing.

they are born with an innate knowledge for music, being able to harmonise/sing/dance together automatically even without any knowledge of music theory, almost like their brains have bluetooth that can connect to each other.

though personalities may vary, a common trait (there are exceptions) is that Yeganis are quite headstrong and passionate

technically all yeganis are instinctively pansexual but respect the concept of sexuality in other cultures because they understand and share the importance of individual identity.

Yeganis, while mostly bipedal, frequently shift to a quadrupedal walk cycle. they often alternate  between 2 legs and 4 legs depending on the situation or how they wish to manoeuvre. when running or walking on four legs, they move in a way that's similar to cheetahs and other cats.

while they are a very compassionate and kind species, showing loving and gentle behaviour toward most living organisms, they are still capable of brutal, animalistic violence due to their unique psychology, brute strength, strong protective instincts (2.5x that of a human), and instincts as a predatory species. 

in the wild (a bit earlier in their evolution but still prominent) Yeganis hunt in groups of up to 6-9 on average, using both ambush and pursuit techniques, intelligently coordinating their attacks.