No it's not. There are branches all over the place.
The whole premise of the link you gave is
Eohippus evolved into Orohippus which then evolved into Pliohippus which then evolved into Modern Horse.
Which is a lie and totally wrong and no biologist claims it happened that way.
What happened was some of Eohippus had decendants that became Propalaeotherium, while some others eventually became Pachynolophus while some others eventually became Orohippus and then the rest of the Eohippus individuals went extinct. Then after many more branches and transitional forms one of many of those branches from Orohippus led to the Merychippus and from the some of the Merychippus decendents branched into Calippus which then branched into Pliohippus while other Merychippus branched into Dinohippus which branched into Hippidion and Equus in which the Equus branched into old world and today's new world horses.
That site you linked disingenously compares the Pliohippus with modern day horses as some how proof that evolution is false when they aren't even directly or as you incorrectly put it "linear" related to each other. They may have a common distant ancestor but they are not related on the same branch.
And as for you assertion that evolution going from simple to complex, Well that's not entirely correct either. Evolution is driven by what survives to have the most offspring that will survive which is why the most abundent lifeforms are simple one cell bacteria.
So again where are all these scholars you claim doubt horse evolution?
Oh and of course where are the Unicorns?
That's the problem. All there seems to be is branches, no roots or trunk anywhere to be seen at all. If you want to claim that horses evolved, you need to show roots and a trunk as well as branches.
The evidence which actually exists could as easily if not more easily be explained by claiming that Jehovah, Wotan, or Allah or somebody simply didn't know what sort of horse would thrive on this planet and therefore created fifty or sixty different kinds of them and let natural selection sort the thing out and weed out all the losers.