I have never had any luck with lasso and whistle. Whistle they usually hate and lasso they hate or it doesn't work. 
I would also like to leran how to use these two items. With the demande that has been on the market, at lest earlier, I would say they are quite popular so they has to have some useful function.

Me personally as well, I never had much luck with whistles and lassos. Petting like you said is a shot in the dark. I tended to go with horses that had wild essence, the colorful bar on the left, be about middle and their background image be yellow or orange. If the bar is any higher paired with orange or red background image? I move on. 

I have seen that non sweet treats and sweet treats go better for me. For example you can take carrots, apples, clover and combine them with Alfalfa and Oats ((I am just waking up so memory is tad iffy but I believe that is how it goes)). You do these under Cooking of the Craft Room Tab. Doing that first round of combining just gives you plain round treats, so Alfalfa clover. Alfalfa apple etc etc. You can take it even a step further and take those plain round treats and make them sweet treats by taking a plain treat and cook it with molasses which gives you sweet treats.

Sweet treats I have found out I do better with wilds. Sometimes they do not always work out and they do run away unfortunately.

What you could do is what I have off and on been doing. Get a word doc going or even use notes on your phone or even a notebook and note down how it goes with the wilds. For example, I had caught a wild thoroughbred mare recently so I would have put down: Adult TB mare. Then use each item I had, but the lassos/whistles and petting. So I would go sweet Alfalfa carrot treat gave her 12% of bond. Next treat woule be sweet oat apple treat gave her 16% bond and just do that until I can call her or if she ran away. Heck even doing back to back sweet oat apple treats could chase them off even though it can give them the highest bond, it just depends on the horse. Even that TB mare, I ran out of plain and sweet treats and ended up just using apples, carrots until I finally got her.

Noting it down small details like that has helped me out in the past but I haven't used it too too much as I mostly didn't have enough stock of my treats I preferred to use and mostly chased the wilds off.

Hope this helps!

For me, it feels like a lot of foals/draft breeds I can bring in with treats and petting.  Paints seem to run the second I pet them, most others run if you give them a second treat.  So unless it's a foal, I just take my shot at spamming the lassos/whistles back and forth and crossing my fingers.  It works like 40% of the time, but it saves energy so I just keep exploring and trying.  

I'll also state that I only try to catch Paints, Thoroughbreds, and Mustangs.  Other breeds I only try to catch if they're interesting looking or show a variation.

Editing to add:  the lassos and whistles will rarely show that they are moving the bar after the first use.  They catch the horse despite the bar not being filled.  I'm not sure if it's a sort of chance system or what.  But I sit and click them as if I'm entering a horse into training, with about a 1s pause. Sometimes I click back and forth between the whistle and lasso.  I know I've caught the horse because you can see a page start loading in chrome, lol.

Thanks Nittrous :)

With 32 lassos and 27 whistles in storage I will try your method a couple of times and see if I have any success. I used treats the other day to catch a Paso Fino mare I really wanted but she decided to run away after use of 3 treats. With the very hard earned ingredients for treats I really don't want to loose them that way. Three completed (sweet treats) is more than I collect ingredients for during a month.

i'm not sure if i've just gotten lucky but i seem to have some luck with using whistles on aggressive wilds (with a red icon) and using pets and treats on calmer ones. doing the opposite tends to scare them away quickly. and yes foals and drafts do seem to take petting and treating much better in my experience.

I tried the whistle lasso approach yesterday... and ended up with a Chincoteague that i have no cliue what to do with.

One thing I noticed was that I didn't create a bond with the horse so it seems that this is following a different path than the treats. Those paths seems incompatible so you have to choose either the reat-petting path or the lasso-whistle path. It can't be intended to work that way?

The wiki states lasso can increase bond but I have heard lasso and or whistle rather lowers their wild essence/makes them less angry.

As for Chinco, you can keep it, auction it off, put it up for sale or retire it. I feel the bubble on the Chinco has burst and no longer as desired as much but that's just my take on it.

I also noticed that the bar on the left decreased useing lasso and whistle, until the horse suddenly apperaed in my stable. 
Earlier when I have used these two they have decreased the bond formed by treats so that's why I feel that the two methods are not compatible