Siamese cat noun

A domestic fawn-coloured cat, with blue eyes and a small head, probably descended from the jungle cat of India, Africa, etc

Siamese fighting fish noun

A brilliantly coloured freshwater fish of SE Asia, the males having elongated sail-like fins and being extremely pugnacious

Siamese twins plural noun

1. Orig a set of Chinese twins (1811–74), born in Siam, joined from birth by a fleshy ligature
2. (also conjoined twins) any set of twins who are physically joined
3. An inseparable pair (figurative)