We've obviously been on numerous treks in Nepal and have seen a couple of Himalayan Tahrs. Generally Nepal has a lot of wildlife, even in Kathmandu city at times. I am giving you a link to a video where a leopard was recently found in Kathmandu: https://www.youtube.com/watch?v=mjtGtYEPCYo
If you go to the national park Chitwan you will easily find tigers, rhinos, elephants etc as well. The park is great but some people may feel it slightly touristy. If you go to Bardia National Park, however, then you will experience untouched habitats and plenty of wildlife there. The downside of going to Bardia is that it is quite far away from Kathmandu city, compared with Chitwan.
Any further questions, please do not hesitate to let us know.
We hope above was helpful.