"This cat is feeling hot."
Translation:इस बिल्ली को गर्मी लग रही है।
Here, इस is not a personal pronoun, but a demonstrative pointing to the cat.
The followings are Hindi demonstratives: -
इस this, उस that, इन these, उन those.
Okay so इस goes in front of a noun. "This is a cat" would be "यह बिल्ली है" whereas "This cat is yellow" would be "इस बिल्ली पीली है" and "This is a yellow cat" would be "यह पीली बिल्ली है" which happens to sounds funny to me (billi pili and pili billi). Excellent, thanks.
यह and वह won't work here. इस will have to be used to mean "this". Thought I can't quite explain why. I hope someone else can properly explain this.