We couldn't find any definitive yes or no on this subject. Apparently, eating enough bluebonnets can be dangerous for a horse, depending on their size, age and how much is eaten. We found several websites that touched on the subject, and have included some quotations from them.
"Normally, horses won't eat poisonous plants. But, during summer months when pasture grasses turn dry and brown or when the pasture is over grazed, horses will eat anything they can find. Often grasses found around irrigated landscape plants or irrigation systems contain dangerous weeds along with those blades of succulent grass. And a young, nosey horse will often try something just to see what it tastes like.
Did you know that the fragrant flowering shrub from which we get perfume - jasmine -is death to horses? So are larkspur, bluebonnet, creeping ivy and buttercup - all popular landscape plants. Even the leaves of oak trees are toxic if eaten in large enough quantities."
Understanding Horse Nutrition from the Bexar County Extension Office:
Are bluebonnets dangerous in a horse pasture?
Bluebonnets are somewhat toxic, but very distasteful to animals. Horses, cows and bluebonnets have co-existed for as long as they have all been around."
"The Moderately Toxic listing includes: Bulbs, Lupine (Bluebonnets!), Rhubarb, Azalea, Rhododendron, Oats, Larkspur, Milkweed, Mustard, Spurges, Nightshades, Black Walnut, and Red Oak."
You will have to come to your own conclusion. We certainly wouldn't suggest cultivating bluebonnets in a pasture meant for horses, but this USDA Plant Profile map does not show Lupinus texensis (Texas bluebonnet) growing natively in the Pearland area anyway.